SecProbe.io

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 September 10, 2025

Jason K. Garland
Chief Financial Officer
Repligen Corporation
41 Seyon Street
Building 1, Suite 100
Waltham, MA 02453

 Re: Repligen Corporation
 Form 10-K for Fiscal Year Ended December 31, 2024
 File No. 000-14656
Dear Jason K. Garland:

 We have completed our review of your filings. We remind you that the
company and
its management are responsible for the accuracy and adequacy of their
disclosures,
notwithstanding any review, comments, action or absence of action by the staff.

 Sincerely,

 Division of Corporation
Finance
 Office of Life Sciences
</TEXT>
</DOCUMENT>

CORRESP
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 CORRESP

 Jacqueline Mercier +1 617 570 1762
 JMercier@goodwinlaw.com

 Goodwin Procter LLP 100 Northern
Avenue Boston, Massachusetts 02210
 goodwinlaw.com +1 617 570 1000
 August 26, 2025 VIA
EDGAR Division of Corporation Finance Office of
Life Sciences Securities and Exchange Commission 100 F
Street, NE Washington, DC 20549 Attention: Mr. Gary
Newberry Mr. Eric Atallah
 Re: Repligen Corporation Form 10-K for the fiscal year ended December 31, 2024 Filed March 14, 2025
 File No. 000-14656
 Dear Mr. Newberry and Mr. Atallah: This letter is submitted on
behalf of Repligen Corporation (the “Company”) in response to the comment of the staff of the Division of Corporation Finance (the “Staff”) of the U.S. Securities and Exchange Commission (the “Commission”)
regarding the Company’s Form 10-K for the fiscal year ended December 31, 2024 (File No. 000-14656), filed on March 14, 2025 (the “Form 10-K”), as set forth in the Staff’s letter dated August 15, 2025 (the “Comment Letter”).
 For reference purposes, the text of the Comment Letter has been reproduced herein with responses below the numbered comment. For your convenience, we have
italicized the Staff’s comments from the Comment Letter. Unless otherwise indicated, page references in the descriptions of the Staff’s comments refer to the Form 10-K. All capitalized terms
used and not otherwise defined herein shall have the meanings set forth in the Form 10-K. Form 10-K for Fiscal Year Ended December 31, 2024 Note 5 - Acquisitions
 2024 Acquisitions, page 88

 1.
 You have identified developed technology and goodwill from the Tantti acquisition totaling $76 million.
On page 14, you refer to patent assets acquired as part of this acquisition and your policy footnote on page 83 refers to the estimated fair value of customer relationships, developed technologies, trademark/tradename, patents, noncompete agreements
and in-process research and development from your acquisitions.
 1

 Given the significance of the allocation to goodwill and developed technology, please
tell us why you have not identified or assigned value to patents or other intangible assets from this acquisition. Refer to ASC 805-20-55. As part of your response,
please include a general description of the procedures you follow to identify intangible assets in your business acquisitions. RESPONSE :
 The Company respectfully advises the Staff that as disclosed in the Form 10-K, the Company’s growth strategy
includes making targeted acquisitions of businesses that have innovative technologies and products. While conducting due diligence on the target’s business and following completion of a business acquisition, the Company conducts a thorough
review of both financial and operating information of the acquiree, including but not limited to historical financial statements, product and customer sales data, significant executory contracts, financial and tax diligence reports, tax returns,
marketing materials, and website information. This information is supplemented through discussions with key members of management of the acquiree. This
information is then analyzed by Company management including members of the finance, legal, and operations departments as well as executive leadership, and where necessary, third party valuation professionals and third party accounting
professionals, to identify assets that lack physical substance and meet either the separability criterion or contractual-legal criterion described in ASC 805-20-55-2 through 55-5. After applying the separability
criterion and the contractual-legal criterion to identify intangible assets, the Company considers the combination of individual intangible assets for purposes of grouping complimentary intangible assets if the assets have similar useful lives and
pattern of consumption of economic benefits, consistent with the guidance in ASC 805-20-55-18. The Company also considers other
information in making the unit of account determination, including the list of factors described in ASC 350-30-35. The Company also considers materiality in making this
assessment. Material intangible assets recognized in the Company’s acquisitions can be primarily characterized as customer-related and
technology-based intangible assets. In certain acquisitions, the Company has identified and recognized contract-based intangible assets based on the Company’s evaluation of contractual terms relative to market terms. Marketing-related
intangible assets, including trademark, tradename and noncomplete agreements are not generally material based on the relative size of the acquirees’ operations and also considering the Company’s plan to continue or integrate
brand-related assets into Repligen’s product portfolio. Artistic-related intangible assets are not typically present in the bioprocessing market.
 The Company followed its customary procedures described above to determine the identifiable intangible assets in the acquisition of Tantti. At the time of
announcing the definitive agreement to acquire Tantti on July 29, 2024, Tantti had planned to launch its first generation chromatography bead product in the second half of 2024. Tantti’s first generation chromatography bead product
commercially launched on December 9, 2024, one week after Repligen’s completion of the acquisition. In connection with the Company’s acquisition of Tantti, the following material identifiable intangible assets were recognized for
accounting purposes:

 1.
 Developed technology : As disclosed in Note 5 in the Form 10-K,
the Company recognized a developed technology intangible asset representing a combined unit of account of all intellectual property (inclusive of patented and unpatented technologies), know-how, business
processes, and technological processes underlying Tantti’s portfolio of microporous chromatography beads. The Company acknowledges that certain individual elements or attributes of the combined developed technology asset might meet the
contractual-legal criterion (e.g., patents) or separability criterion,
 2

however these individual elements or attributes are highly interrelated with the other individual elements or attributes of the combined developed technology asset. Management considered the
guidance in ASC 350-30-35-23 through 35-24 and 805-20-55-18 in making this determination, noting that the combined developed technology asset, and not the individual elements or attributes, represents the highest
and best use of the developed technology assets and the period of economic benefit of the individual elements or attributes is similar. The generation of cash flows for Tantti is driven by the combined unit of account, which is disclosed as
“Developed Technology”. Based on its customary procedures described above to determine the identifiable intangible assets in
the acquisition of Tantti, the Company determined the following types of intangible assets were either not material to the consolidated financial statements and therefore not recognized, or did not meet either the separability criterion or
contractual-legal criterion under ASC 805-20-55-2 through 55-5:

 2.
 Customer relationships : As of the acquisition date of December 2, 2024, Tantti had not commercially
launched its first generation chromatography bead product and Tantti had no commercial sales history. Accordingly, there was no customer list and no prior history of establishing commercial revenue agreements via contract (that is, no sales orders
or purchase orders or order backlogs arising from contracts) at the acquisition date. The first generation chromatography bead product launched commercially on December 9, 2024, one week after completion of the acquisition by the Company. Based
on these facts and circumstances, the Company determined no customer-related intangible assets met the contractual-legal criterion or the separability criterion as of the acquisition date. As described further below with respect to IPR&D,
development of the first generation product was substantially complete as of the acquisition date.

 3.
 Trademark/tradename : The Company acquired certain tradenames registered in various jurisdictions
including China, Taiwan, Japan, Europe, and the United States, noting that the legal registration indicates legal protection as discussed in ASC
 805-20-55-17, and thus these tradenames meet the contractual-legal criterion. The Company engaged a third-party valuation
professional to determine the fair value of the tradenames at acquisition, concluding the value of such asset was less than $0.7 million. Management concluded this amount was not material to the Company’s consolidated financial statements
or to the users of the financial statements and did not recognize a separate intangible asset as a result.

 4.
 Noncompete agreements : Certain employees of the acquiree entered into noncompete agreements with the
Company in contemplation of the acquisition and such noncompete agreements became effective upon the acquisition closing date. The Company evaluated these contracts under the marketing-related intangible assets guidance in ASC 805-20-55-16 through 55-18. The Company engaged a third-party valuation professional to
determine the fair value of the noncompete agreements at acquisition, concluding the value of such asset was less than $0.2 million. Management concluded this amount was not material to the Company’s consolidated financial statements or
to the users of the financial statements and did not record a separate intangible asset as a result.

 5.
 In-process research and development (“IPR&D”) :
As described in the customer relationship evaluation, there were no substantive incomplete research and development projects ongoing as of the acquisition date. As there were no remaining commercialization or development risks with respect to
Tantti’s first generation chromatography bead product requiring more than insignificant costs to be incurred by Repligen, Tantti’s research and development projects were considered to be complete for accounting purposes and recognition
of IPR&D in the application of ASC 805 would have been inappropriate. Had Tantti’s research and development projects been substantive and in-process as of the acquisition date, the Company would have
recognized an IPR&D asset rather than a developed technology asset as part of its application of ASC 805 following the process described above.
 3

 In addition to these intangible assets, as part of the acquisition, the Company considered whether any other
categories of intangible assets not listed above could be present, and none were identified. The Company acknowledges the Staff’s comment with
respect to disclosures on page 83 of the Form 10-K and will update its disclosures to more clearly define the recognized intangible assets in its business acquisitions in future filings.
 Beginning with our Form 10-Q for the quarter ending September 30, 2025, we will update the business combinations
accounting policy disclosure in the “Summary of Significant Accounting Policies” footnote to read as follows: Business
Combinations Total consideration transferred for acquisitions is allocated to the tangible and intangible assets acquired and
liabilities assumed, if any, based on their fair values at the dates of acquisition[…] The Company typically uses the income
approach to determine the fair value of certain identifiable intangible assets including customer relationships and developed technology. This approach determines fair value by estimating after-tax cash flows
attributable to these assets over their respective useful lives and then discounting these after-tax cash flows back to a present value. The Company bases its assumptions on estimates of future cash flows,
expected growth rates, expected trends in technology, etc. Discount rates used to arrive at a present value as of the date of acquisition are based on the time value of money and certain industry-specific risk factors. The Company believes the
estimated purchased customer relationships, developed technologies, trademark/tradename and other intangible assets identified in its acquisitions represent the fair value at the date of acquisition, and do not exceed the amount a third-party would
pay for such assets. If you should have any questions or comments with respect to the foregoing, please contact me at
 617-570-1762 or via e-mail at jmercier@goodwinlaw.com.

 Very truly yours,

 /s/ Jacqueline Mercier

 Jacqueline Mercier

 Goodwin Procter LLP

 cc:
 Olivier Loeillot, Chief Executive Officer of Repligen
 Jason K. Garland, Chief Financial Officer of Repligen
 Marc Baumgartner, Controller at Repligen
 Brittany Morreale, Goodwin Procter LLP
 4

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<TEXT>
 August 14, 2025

Jason K. Garland
Chief Financial Officer
Repligen Corporation
41 Seyon Street
Building 1, Suite 100
Waltham, MA 02453

 Re: Repligen Corporation
 Form 10-K for Fiscal Year Ended December 31, 2024
 File No. 000-14656
Dear Jason K. Garland:

 We have limited our review of your filing to the financial statements
and related
disclosures and have the following comment.

 Please respond to this letter within ten business days by providing the
requested
information or advise us as soon as possible when you will respond. If you do
not believe a
comment applies to your facts and circumstances, please tell us why in your
response.

 After reviewing your response to this letter, we may have additional
comments.

Form 10-K for Fiscal Year Ended December 31, 2024
Note 5 - Acquisitions
2024 Acquisitions, page 88

1. You have identified developed technology and goodwill from the Tantti
acquisition
 totaling $76 million. On page 14, you refer to patent assets acquired as
part of this
 acquisition and your policy footnote on page 83 refers to the estimated
fair value of
 customer relationships, developed technologies, trademark/tradename,
patents, non-
 compete agreements and in-process research and development from your
acquisitions.

 Given the significance of the allocation to goodwill and developed
technology, please
 tell us why you have not identified or assigned value to patents or
other intangible
 assets from this acquisition. Refer to ASC 805-20-55. As part of your
response,
 please include a general description of the procedures you follow to
identify
 intangible assets in your business acquisitions.
 August 14, 2025
Page 2

 In closing, we remind you that the company and its management are
responsible for
the accuracy and adequacy of their disclosures, notwithstanding any review,
comments,
action or absence of action by the staff.

 Please contact Gary Newberry at 202-551-3761 or Eric Atallah at
202-551-3663 with
any questions.

 Sincerely,

 Division of
Corporation Finance
 Office of Life
Sciences
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August 12, 2024
Jason K. Garland
Chief Financial Officer
Repligen Corp
41 Seyon Street
Bldg. 1, Suite 100
Waltham , MA 02453
Re:Repligen Corp
Form 10-K for the fiscal year ended December 31, 2023
Filed February 22, 2024
File No. 000-14656
Dear Jason K. Garland:
            We have completed our review of your filing. We remind you that the company and its
management are responsible for the accuracy and adequacy of their disclosures, notwithstanding
any review, comments, action or absence of action by the staff.
Sincerely,
Division of Corporation Finance
Office of Life Sciences

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 Goodwin Procter LLP

 100 Northern Ave.,

Boston, MA 02110

goodwinlaw.com

 +1 (617)
570-1000

 August 6, 2024

 VIA
EDGAR

 Division of Corporation Finance

 Office of
Life Sciences

 Securities and Exchange Commission

 100 F
Street, NE

 Washington, DC 20549

Attention:
 Ms. Mary Mast

Ms. Ibolya Ignat

Re:
 Repligen Corporation

 
 Form 10-K for the fiscal year ended December 31, 2023

 
 Filed February 22, 2024

 
 File No. 000-14656

Dear Ms. Mast and Ms. Ignat:

 This letter is submitted
on behalf of Repligen Corporation (the “Company”) in response to the comments of the staff of the Division of Corporation Finance (the “Staff”) of the U.S. Securities and Exchange Commission (the “Commission”) regarding
the Company’s Form 10-K for the fiscal year ended December 31, 2023 (File No. 000-14656), filed on February 22, 2024 (the “Form 10-K”), as set forth in the Staff’s letter dated July 24, 2024 (the “Comment Letter”).

 For
reference purposes, the text of the Comment Letter has been reproduced herein with responses below the numbered comment. For your convenience, we have italicized the Staff’s comments from the Comment Letter. Unless otherwise indicated,
page references in the descriptions of the Staff’s comments refer to the Form 10-K. All capitalized terms used and not otherwise defined herein shall have the meanings set forth in the Form 10-K.

 The responses provided herein are based upon information provided to Goodwin Procter LLP by the Company.

Form 10-K for the year ended December 31, 2023

Notes to the Financial Statements

 14. Convertible
Senior Notes, page 114

1.
 We note that your Convertible Notes are convertible at the option of the holder and that the make-whole
fundamental change provision may trigger an increase in the conversion rate. Please explain to us how you evaluated the Notes to determine whether each conversion feature was required to be bifurcated and accounted for as a derivative under ASC 815.
In your response, please specifically address whether each conversion option contingency meets the definition of a derivative and, if so, whether it qualifies for the scope exception for contracts involving an entity’s own equity as set forth
in ASC 815- 10-15-74(a). Specifically address the steps in ASC 815-40-15-7A through
15-7H, the conditions beginning in ASC 815-40-25-7, and if the make-whole provision
violates the condition in ASC 815-40-25-39. Please also revise your accounting policy disclosure accordingly in your future
filings.

  Page
 2

 RESPONSE:

The Company notes that while the discussion below is focused on the 2023 Notes, the same analysis is applicable to the 2019 Notes which have equivalent terms.

 The Company assessed the embedded conversion feature to determine if bifurcation from the 2023 Notes and derivative accounting pursuant to ASC 815-15-25-1 is required. As part of the analysis, the embedded conversion feature was assessed to determine if it meets the definition
of a derivative and qualifies for a derivative scope exception. This analysis included evaluating all contingent exercise provisions and settlement provisions of the 2023 Notes.

The 2023 Notes may be converted in multiple different situations related to (i) trading price of the 2023 Notes, (ii) the Company’s issuance of
certain securities or rights, (iii) upon the occurrence of defined corporate events (which includes a make-whole fundamental change), (iv) common stock price target trigger, (v) in connection with the Company’s redemption option, and (vi) non-contingently after the free convertibility period. Because of the commonality of the underlying (i.e., settlement price is always based on the value of equity) irrespective of the triggering events
(and corresponding settlement provisions), the Company determined the conversion feature represents a single option with multiple exercise triggers. Thus, the conversion feature represents a single unit of analysis for accounting purposes.

Additionally, if a holder converts upon a make-whole fundamental change (representing one of the defined corporate event exercise triggers described above) or
in connection with the Company’s redemption option, the holder may be eligible to receive a make-whole premium through an increase to the conversion rate. The make-whole premium was designed to compensate the holder for lost
“time-value” of the conversion option (i.e., the difference between the conversion option’s fair value and intrinsic value) by using a standard option pricing tool. There were no inputs to the construction of the make-whole table
other than standard, permitted option pricing inputs.

 As discussed in further detail, the Company respectfully advises that the conversion feature meets
the scope exception to the derivative accounting guidance in ASC 815-10-15-74(a) and does not meet the bifurcation criteria in
ASC 815-15-25-1(c). Therefore, the Company concluded that the conversion feature should not be accounted for as a derivative.

 Accounting Analysis – 2023 Notes

 The
Company first concluded the conversion feature meets the definition of a derivative instrument consistent with ASC 815-10-15-83,
as the feature has an underlying, notional or payment provision, no initial net investment and provide for net settlement.

 Under the scope exception
pursuant to ASC 815-10-15-74(a), a reporting entity is not required to treat a contract issued or held by that reporting entity
as a derivative instrument if the contract is both (1) indexed to its own stock and (2) classified in stockholders’ equity in its statement of financial position. If the conversion feature qualifies for this derivative scope
exception, bifurcation and separate derivative accounting is not required. Both criteria are evaluated below:

 (1). Indexed to reporting entity’s
common stock – ASC 815-40-15-7 requires that companies apply a two-step approach
in evaluating whether an equity-linked financial instrument or embedded feature is indexed to its own stock. The first step requires an evaluation of the instrument’s contingent exercise provisions, if any. The second step requires an
evaluation of the instrument’s settlement provisions.

  Page
 3

 Step 1: ASC 815-40-15-7(a) — Evaluate the instrument’s contingent exercise provisions, if any.

 Under
ASC 815-40-15-7A, an exercise contingency does not preclude an instrument (or embedded feature) from being considered indexed to
an entity’s own stock if it is not based on either (a) an observable market, other than the market for the issuer’s stock (if applicable) or (b) an observable index, other than an index calculated or measured solely by reference
to the issuer’s own operations.

 The conversion feature contains standard contingent exercise provisions, which include (i) trading price of the
2023 Notes, (ii) the Company’s issuance of certain securities or rights, (iii) upon the occurrence of defined corporate events (which includes a make-whole fundamental change), (iv) common stock price target trigger (v) in
connection with the Company’s redemption option, and (vi) non-contingently after the free convertibility period. The contingent exercise provisions of the 2023 Notes are not based on an observable market or index that is not based on the
Company’s operations, and thus do not preclude the conversion feature from being indexed to the Company’s own stock. Step 2 of the indexation guidance was next considered.

Step 2: ASC 815-40-15-7(b)
— Evaluate the instrument’s settlement provisions.

 The second step is evaluating whether the settlement provisions are consistent with a
fixed-for-fixed equity instrument (fixed conversion price). This includes evaluating the increase in the settlement provisions of the 2023 Notes under a make-whole
fundamental transaction. The 2023 Notes have the following settlement provisions whereby the conversion rate may be adjusted:

•

 Anti-dilution adjustment provisions to the conversion rate;

•

 Voluntary increase in the conversion rate; and

•

 Conversion rate adjustment upon occurrence of (i) a make-whole fundamental change or (ii) holder
conversion in relation to the exercise of the Company redemption option.

 The settlement provisions were evaluated under ASC 815-40-15-7C through 15-7H, as summarized below:

Anti-dilution provision. The 2023 Notes include a standard anti-dilution provision that protects the holder’s exposure to the risks arising from
certain events such as a stock split or a dividend. ASC 815-40-15-7E provides that adjustments to the conversion price as a
result of commercially reasonable anti-dilution provisions and other corporate events would not preclude the instrument from being indexed to an entity’s own stock. The 2023 Notes include standard anti-dilution provisions that adjusts the
conversion price to offset the impact of the events noted above. Each of these adjustments are consistent with the guidance for equity accounting illustrated by Example 17 of ASC
815-40-55. As such, the anti-dilution provision meets the criterion in ASC 815-40-15-7E and does not preclude the conversion feature from being considered indexed to the Company’s own stock.

Voluntary increase in the conversion rate. The 2023 Notes include a provision whereby the Company may voluntarily increase the conversion rate at the
discretion of the Company’s Board of Directors. ASC 815-40-15-7H provides that if an equity-linked financial instrument
includes terms that allow the issuer to modify the instrument at any time provided such modifications benefit the counterparty, then these terms do not preclude the instrument from being considered indexed to the entity’s own stock. The
increase in the conversion rate benefits the 2023 Notes holder (i.e., counterparty) thus, the Company concluded that such provision does not preclude the conversion feature from being considered indexed to an entity’s own stock.

  Page
 4

 Make-whole fundamental change premium. The 2023 Notes include a make-whole provision that adjusts the
conversion rate upon the occurrence of a fundamental change or upon the exercise of the Company redemption option whereby the holder may be eligible to receive a make-whole premium through an increase to the conversion rate. The Company evaluated
the make-whole provision under ASC 815-40-15-7C through 15-8 to determine whether the
equity-linked feature can be considered indexed to the Company’s own stock. Under ASC 815-40-15-7D, the make-whole feature
would still be considered indexed to the Company’s own stock if the only variables that could affect the settlement amount would be inputs to the fair value of a
fixed-for-fixed forward or option on equity shares. Under ASC
815-40-15-7F, the make-whole feature would not be considered indexed to the Company’s own stock if its settlement amount is
affected by variables that are extraneous to the pricing of a fixed-for-fixed option or forward contract on equity shares.

The make-whole provision in the 2023 Notes adjusts the conversion rate to compensate the holders of the notes for lost time value as a result of an early
settlement. The number of additional shares is determined based on a table with axes of stock price and time, which would both be inputs in a fair value measurement for a
fixed-for-fixed option on equity shares. The make-whole table was designed such that the aggregate fair value of the consideration deliverable (including the cash paid
for principal and, at the Company’s option, cash or shares delivered for the conversion premium, that is, the fair value of the underlying shares per bond plus the make-whole shares) would be expected to approximate the fair value of the
convertible debt instrument at the settlement date, subject to market changes in relevant option pricing inputs, assuming no change in relevant pricing inputs (other than stock price and time) since the instrument’s inception. The make-whole
provision adjustment within the 2023 Notes is consistent with Example 19 in ASC 815-40-55-45 through 46 wherein it is concluded
that the adjustment to the settlement amount due to the make-whole adjustment does not preclude the conversion option from being deemed indexed to the Company’s own stock. As a result, the provision for adjustments based on this make-whole
table does not preclude the conversion feature from being considered indexed to the Company’s own stock.

 As the only variables that could affect the
settlement amount would be inputs to the fair value of a fixed-for-fixed forward or option on equity shares, the settlement adjustments, including the make-whole
feature, included in the 2023 Notes do not preclude the conversion feature from being considered indexed to the Company’s own stock under ASC
815-40-15-7D.

 (2).
Classified in stockholders’ equity in its statement of financial position – The Company highlights that the principal (and accrued interest) balance of the 2023 Notes is only settled in cash, and the Company has the option to settle
the conversion feature in cash, common stock shares or a combination of the two (e.g., “Instrument C’’, as defined within EITF Issue 90-19.). ASC 815-40-25 states that to determine whether the embedded conversion feature would be classified in stockholders’ equity, the reporting entity must consider who controls the settlement and whether the
settlement will be in shares or cash. ASC 815-40-25-2 states that if the contract provides the entity with the choice of net cash
settlement or settlement in shares, the settlement in shares is assumed and contracts that require settlement in shares are equity instruments.

 The
condition in ASC 815-40-25-39 states that guidance in ASC 815-40-25-7 through 25-30 would not apply if the holder may only realize the value of the conversion option by exercising the option and receiving the entire
proceeds in a fixed number of shares or the equivalent amount of cash (at the discretion of the issuer). The 2023 Notes do not meet this exception because the principal portion of the 2023 Notes is settled in cash and the make-whole provision is
designed so that the holder of such notes would receive the approximate fair value of the notes at the settlement date and the Company has the option to settle the conversion option in cash, shares or a combination of the two. Therefore, the Company
assessed the additional provisions beginning in ASC 815-40-25-7 through 25-35 and
determined that all of the criteria have been met and, as such, the conversion option should be classified in stockholders’ equity under ASC 815-40-25. The
Company’s analysis is outlined below with respect to key conditions of ASC 815-40-25:

  Page
 5

•

 There is no explicit requirement to settle the contract in cash;

•

 The Company has sufficient authorized and unissued shares;

•

 The 2023 Notes contain an explicit share limit;

•

 There is no requirement to cash settle the contract if the Company fails to make timely filings with the SEC; and

•

 There is no cash-settled top-off or make whole provision as described in
ASC 815-40-25-30.

As the conversion feature is also deemed indexed to the Company’s own stock, the scope exception available under ASC 815-10-15-74 is met and bifurcation from the 2023 Notes is not required.

Accounting Analysis – 2019 Notes

 The Company
completed a similar accounting analysis with respect to convertible notes issued on July 15, 2019 (“2019 Notes”), The Company assessed the embedded conversion feature to determine if bifurcation from the 2019 Notes and derivative
accounting pursuant to ASC 815-15-25-1 is required. As part of the analysis, the embedded conversion feature was assessed to
determine if it meets the definition of a derivative and qualifies for a derivative scope exception.

 The conversion feature and settlement provisions of
the 2019 Notes are similar to those contained in the 2023 Notes. The 2019 Notes may be converted in multiple different situations related to

July 24, 2024
Jason K. Garland
Chief Financial Officer
Repligen Corp
41 Seyon Street
Bldg. 1, Suite 100
Waltham , MA 02453
Re:Repligen Corp
Form 10-K for the fiscal year ended December 31, 2023
Filed February 22, 2024
File No. 000-14656
Dear Jason K. Garland:
            We have limited our review of your filing to the financial statements and related
disclosures and have the following comment.
            Please respond to this letter within ten business days by providing the requested
information or advise us as soon as possible when you will respond. If you do not believe
the comment applies to your facts and circumstances, please tell us why in your response.
            After reviewing your response to this letter, we may have additional comments.
Form 10-K for the year ended December 31, 2023
Notes to the Financial Statements
14. Convertible Senior Notes, page 114
1.We note that your Convertible Notes are convertible at the option of the holder and
that the make-whole fundamental change provision may trigger an increase in the
conversion rate.  Please explain to us how you evaluated the Notes to determine whether
each conversion feature was required to be bifurcated and accounted for as a derivative
under ASC 815.  In your response, please specifically address whether each conversion
option contingency meets the definition of a derivative and, if so, whether it qualifies for
the scope exception for contracts involving an entity's own equity as set forth in ASC 815-
10-15-74(a).  Specifically address the steps in ASC 815-40-15-7A through 15-7H, the
conditions beginning in ASC 815-40-25-7, and if the make-whole provision violates the
condition in ASC 815-40-25-39.  Please also revise your accounting policy disclosure
accordingly in your future filings.

July 24, 2024
Page 2
            In closing, we remind you that the company and its management are responsible for the
accuracy and adequacy of their disclosures, notwithstanding any review, comments, action or
absence of action by the staff.
            Please contact Ibolya Ignat at 202-551-3636 or Mary Mast at 202-551-3613 with any
questions.
Sincerely,
Division of Corporation Finance
Office of Life Sciences

Mail Stop 4546
March 8, 2017

Via E -mail
Mr. Jon Snodgres
Chief Financial Officer
Repligen Corporation
41 Seyon Street
Bldg. 1, Suite 100
Waltham, MA  0245 3

Re: Repligen Corporation
 Form 10-K for the fiscal year ended December 31, 2015
Filed February 2 5, 2016
File No. 0 -14656

Dear Mr. Snodgres :

We have completed our review of your filing.  We remind you that the company and its
management are responsible for the accuracy and adequacy of the ir disclosure s, notwithstanding
any review, comments, action or absence of action by the staff .

Sincerely,

 /s/ Angela M. Connell

Angela M. Connell
Account ing Branch Chief
Office of Healthcare and Insurance

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filename1.htm

CORRESP

CERTAIN PORTIONS OF THIS LETTER HAVE BEEN OMITTED FROM THE VERSION FILED VIA EDGAR. CONFIDENTIAL
TREATMENT HAS BEEN REQUESTED WITH RESPECT TO THE OMITTED PORTIONS. INFORMATION THAT WAS OMITTED IN THE EDGAR VERSION HAS BEEN NOTED IN THIS LETTER WITH A PLACEHOLDER IDENTIFIED BY THE MARK “[***].”

February 14, 2017

Via EDGAR

 Securities and Exchange Commission

Division of Corporation Finance

 100 F Street, N.E.

Washington, DC 20549-3720

Re:
Repligen Corporation

 Form 10-K for year ended December 31, 2015

Filed February 25, 2016

File No. 0-14656

 Ladies and Gentlemen:

 This letter is submitted by Repligen Corporation (the “Company”, “we”, or “our”)
in response to comments of the staff of the Division of Corporation Finance (the “Staff”) of the Securities and Exchange Commission (the “Commission”) with respect to the Company’s Form 10-K for the fiscal year
ended December 31, 2015 (the “Form 10-K”) as set forth in your letter, dated December 22, 2016 (the “Comment Letter”), and as later conveyed during a discussion in follow-up to the Company’s
submission of its response letter to the Commission on January 24, 2017 (the “Original Response”).

 For
reference purposes, the Staff’s comment has been reproduced in italics with the Company’s response below.

 Financial Statements

Segment Reporting, page 65

 1.
Please provide us, for the fiscal years 2015 and 2014 and for the nine months ended September 30, 2016, product revenues for:

•

Protein A ligands;

•

OPUS® pre-packed chromatography column line;

•

Protein A chromatography resins;

•

LONG® R3 IGF-1; and

•

ATF System

 Securities and Exchange Commission

February 14, 2017

  Page
 2

 as discussed under “Our Products” on page 3. Also, tell us your consideration
for providing product revenue disclosure pursuant to ASC 280-10-50-40.

 Modified Response to Comment No. 1:

The Company acknowledges the Staff’s comment and wishes to further clarify and modify the Original Response.

The manufacturing process for biologic drugs is standardized throughout the industry and consists of three fundamental phases. First, upstream
manufacturing involves the growing, through fermentation, of the biologic from living cells that are grown in a bioreactor under controlled conditions. These cells, which are the factories for producing the biologic drugs, are highly sensitive to
the conditions under which they grow, including the composition of the cell culture media and the growth factors used to stimulate increased cell growth and production of the drug. In the second, downstream step, the drug must be separated and
purified, typically through various filtration and chromatography steps. In the third stage of the process, the purified drug is formulated and packaged into its final injectable form.

The Company is focused on the development, production and commercialization of products that are used in the interconnected phases of the
biologic drug manufacturing process. We utilize common management and practices across all factories and product lines, with equal emphasis, to drive the operating performance of the Company.

 Source: Investor Deck (www.repligen.com):

1.
Acquired SIUS Filtration Technology as part of TangenX acquisition on December 15, 2016 – integrated into Filtration Portfolio – ATF and SIUS.

2.
Acquired Atoll GmbH on April 4, 2016 – integrated into Chromatography Portfolio – OPUS and OPUS PD.

 Securities and Exchange Commission

February 14, 2017

  Page
 3

 Our products are designed and manufactured to support common biologic drug manufacturing
processes across our entire customer base. We broadly market to a common set of end users and we are organized with a focus on our full portfolio to drive the success of the Company.

Following our telephonic discussion with the Staff on February 8, 2017, and to provide users of our financial statements with additional
information regarding our product offerings, we propose to disaggregate our product portfolio for purposes of our annual entity-wide revenue disclosures. We propose to separately disclose revenue from the following groups of similar products in our
annual financial statements:

1)
Filtration Products: Includes our XCell ATF System and Sius filtration products. Together, these two products make up the filtration portfolio of the Company. The design components and use of these
products in the filtration of biologic drugs are substantially similar. Filtration products, which consist of a housing with a filter inside, are used throughout the biologic drug manufacturing process, from the bioreactor to the final concentration
and formulation steps. Filtration devices manufactured under both the XCell ATF and Sius lines consist of filtration “houses” which contain filters that are used to either eliminate specific waste products from the drug or concentrate the
drug. Because all of our filtration products share these common components parts and because their purposes in the biologic drug manufacturing process are similar, we believe they are similar products under ASC 280-10-50-40.

2)
Chromatography Products: Includes our OPUS pre-packed chromatography columns, our proprietary Protein A chromatography resins and our ELISA testing kits, all of which are used in the purification of biologic
drugs. We often package these three component parts together for sale to end users, with the packaged product consisting of the OPUS column, the Protein A resins inside the column, and the ELISA test kit to confirm the column/resin purified the
drug to specifications. Because all three of these products are critical to rendering drugs pure and because our revenue from these products are generally interrelated, we believe they are similar products under ASC 280-10-50-40.

3)
Protein Products: Includes our two protein products, Protein A ligands and LONG®R3 IGF-1 growth factors. Our protein products are developed and
produced using E.coli fermentation and both are sold as powders or liquids. We sell both of these products under long-term supply contracts to multinational bioprocessing enterprises. Both of these proteins are used by customers as additives or
components in chromatography resin manufacturing and cell culture media formulation. As a result, we believe our protein products are similar products under ASC 280-10-50-40.

 Securities and Exchange Commission

February 14, 2017

  Page
 4

 As the Company grows and expands we will add new products into our filtration, chromatography
or protein groupings. In addition to our organically developed products, the assets that we have acquired in recent years through strategic acquisitions complement these product groupings. Our ATF System and Sius lines, which we acquired from Refine
Technology and TangenX, respectively, have helped to expand our filtration product group, while our Atoll GmbH acquisition has further developed our chromatography offerings. We believe that that separately disclosing revenue from Filtration
Products, Chromatography Products and Protein Products will provide more clarity for investors and will provide additional information to users of our financial statements when evaluating the contribution of these similar groupings to our overall
product revenue.

 The Company’s product revenues for its Filtration Products, Chromatography Products and Protein Products groups are
reflected in Annex A attached hereto. Because of the financially sensitive nature of the information set forth in Annex A, the Company requests confidential treatment under 17 C.F.R. § 200.83 of such information and has submitted a separate
request for confidential treatment in accordance therewith to the Commission’s Office of Freedom and Information Privacy Act Operations. Pursuant to Rule 24b-2 under the Securities Exchange Act of 1934, as amended (the “Exchange
Act”), the information set forth in Annex A is being provided to the Commission on a confidential supplemental basis only and is not to be filed with or deemed part of the Exchange Act filings or any other filing made by the Company. The
Company respectfully requests that the Staff return the unredacted version of this letter to it pursuant to Rule 24b-2 of the Exchange Act once the Staff has completed its review. We have provided a self-addressed stamped envelope for this purpose.
Kindly acknowledge receipt of this letter by stamping the enclosed copy of this letter and returning it in the envelope provided.

 Should
you have any further comments or questions with regard to the foregoing, please do not hesitate to contact the undersigned by phone at 781-419-1862 or by e-mail at jsnodgres@repligen.com.

Sincerely,

/s/ Jon Snodgres

 Name: Jon Snodgres

 Title: Chief Financial
Officer

Cc:
Tony J. Hunt

 Repligen Corporation

Arthur R. McGivern

Courtney M. Gaughan

 Goodwin Procter LLP

 Securities and Exchange Commission

February 14, 2017

  Page
 5

 ANNEX A

The following table represents the Company’s total revenue by similar product offering:

(in thousands)

Nine Months
ended
September 30,

Twelve Months ended December 31,

2016

2015

2014

 Proteins

$
[***
]

$
[***
]

$
[***
]

 Filtration

[***
]

[***
]

[***
] (1)

 Chromatography

[***
] (2)

[***
]

[***
]

 Other

[***
]

[***
]

[***
]

 Total Product Revenue

$
[***
]

$
[***
]

$
[***
]

(1)
[***]

(2)
[***]

CONFIDENTIAL TREATMENT REQUESTED BY REPLIGEN CORPORATION

CORRESP
1
filename1.htm

CORRESP

CERTAIN PORTIONS OF THIS LETTER HAVE BEEN OMITTED FROM THE VERSION FILED VIA EDGAR. CONFIDENTIAL TREATMENT HAS BEEN
REQUESTED WITH RESPECT TO THE OMITTED PORTIONS. INFORMATION THAT WAS OMITTED IN THE EDGAR VERSION HAS BEEN NOTED IN THIS LETTER WITH A PLACEHOLDER IDENTIFIED BY THE MARK “[***].”

January 24, 2017

 Via
EDGAR

 Securities and Exchange Commission

 Division
of Corporation Finance

 100 F Street, N.E.

 Washington, DC
20549-3720

Re:

Repligen Corporation

Form 10-K for year ended December 31, 2015

Filed February 25, 2016

File No. 0-14656

 Ladies and Gentlemen:

This letter is submitted by Repligen Corporation (the “Company”) in response to comments of the staff of the Division
of Corporation Finance (the “Staff”) of the Securities and Exchange Commission (the “Commission”) with respect to the Company’s Form 10-K for the fiscal year ended December 31, 2015 (the “Form
10-K”) as set forth in your letter, dated December 22, 2016 (the “Comment Letter”).

 For reference
purposes, the Staff’s comment has been reproduced in italics with the Company’s response below.

 Financial Statements

Segment Reporting, page 65

 1.
Please provide us, for the fiscal years 2015 and 2014 and for the nine months ended September 30, 2016, product revenues for:

•

Protein A ligands;

•

OPUS® pre-packed chromatography column line;

•

Protein A chromatography resins;

•

LONG®R3 IGF-1; and

•

ATF System

 Securities and Exchange Commission

January 24, 2017

  Page
 2

 as discussed under “Our Products” on page 3. Also, tell us your consideration
for providing product revenue disclosure pursuant to ASC 280-10-50-40.

 Response to Comment No. 1:

The Company acknowledges the Staff’s comment and advises the Staff that it considered and evaluated the requirements of ASC 280-10-50-40,
which require a public entity to report revenues from external customers for each product and service or each group of similar products and services, unless it is impracticable to do so. The Company reports revenues from sales of its bioprocessing
products on an aggregated basis as it believes these products constitute a group of similar products pursuant to ASC 280-10-50-40.

 In
determining that its bioprocessing products constitute a group of similar products, the Company considered the following factors:

•

The Company’s products are designed and manufactured to support common bioprocess manufacturing processes across its entire biopharmaceutical and contract manufacturing organization (CMO) customer base, and all of
these products are used or consumed by bioprocessing manufacturing organizations, specifically for the development production of Active Pharmaceutical Ingredients (APIs).

For example, Repligen customers have very consistent and standard manufacturing processes which are subject to approval by the U.S. Food and
Drug Administration (FDA) or other regulatory authorities prior to commencing commercial sales of their drugs. All biological drugs, or APIs, have a fermentation step to produce or grow the drug in cells and subsequent filtration and purification
steps to purify the drug to make it safe for injection into humans. The Company sells to its customers’ manufacturing and development departments who all follow this consistent manufacturing process, regardless of scale.

•

The Company’s products are broadly marketed to a common set of end-user bioprocessing manufacturing organizations of major biopharmaceutical companies and CMOs, as opposed to being marketed to distinct subsets of
the end-user customer base.

 For example, Repligen sells to manufacturing customers who are either developing or scaling
their processes from preclinical manufacturing to Phase I, II, III or full scale commercial manufacturing. CMOs get their directions on the manufacturing of the drug from the manufacturing organizations within their biopharmaceutical company
clients.

•

Opportunities and risks for growth and share gain apply equally to all of the Company’s bioprocessing products. As biologic drugs, particularly monoclonal antibodies, enter Phase I, II or III trials, or are
approved and enter into full scale commercial production processes, all of the Company’s product lines have similar opportunities to be selected and used by its customers.

 Securities and Exchange Commission

January 24, 2017

  Page
 3

 For example, our customers typically test and evaluate our products during their development
phases and our products stay with the manufacturing process as it moves through clinical trials and eventually reaching the commercial phase, which requires FDA approval. As customers move from preclinical, the scale of the operation increases and
the demand for our products increases. This results in more demand for, and increased revenues from, our products.

•

Revenues generated from the Company’s bioprocessing products are similarly impacted by trends in the strength of the overall biologic drug market, economic conditions, and execution of the Company’s growth
strategies, as reflected by strong sales growth in all of the Company’s major products in 2015, a year in which the monoclonal market experienced significant expansion, with the filing of the first biosimilar monoclonal antibody with the FDA,
and a historic high of nine FDA approvals of monoclonal antibodies.

•

All of the Company’s bioprocessing products are subject to, and evaluated, based on similar global regulatory and performance requirements. Our customers’ clinical-stage product candidates and commercial
products are always manufactured in cGMP settings and the critical performance features of all Repligen’s products have been validated and documented. These documents are reviewed for suitability and filed with regulatory authorities. Prior to
commercial adoption, Repligen will be audited by its customers Quality Assurance departments, the results of which are disclosed to the regulatory authorities globally. These manufacturing processes also require a separate approval by the FDA or
other regulatory authorities prior to commencing commercial sales. Therefore, all Repligen’s products have the similar risks associated with customer adoption from a quality and regulatory perspective.

•

The Company utilizes common management and management practices across all factories and product lines, with equal emphasis, to drive the operating performance of the Company.

For example, on our commercial sales team, the Company’s Vice President of Bioprocessing Sales and Marketing is responsible for all
products and each member of his team of sales representatives is responsible for selling and marketing all of the Company’s products.

Based on the end user market, economic, manufacturing and regulatory similarities of the Company’s bioprocessing products and the
commonality of end-user bioprocessing manufacturing organizations, the Company believes it has appropriately considered and disclosed its product revenues in accordance with the guidance of ASC 280.

 Securities and Exchange Commission

January 24, 2017

  Page
 4

 In response to the Staff’s inquiry, the Company’s product revenues for the fiscal
years 2015 and 2014 and for the nine months ended September 30, 2016 are reflected in Annex A attached hereto. Because of the financially sensitive nature of the information set forth in Annex A, the Company requests confidential treatment
under 17 C.F.R. § 200.83 of such information and has submitted a separate request for confidential treatment in accordance therewith to the Commission’s Office of Freedom and Information Privacy Act Operations. Pursuant to Rule 24b-2 under
the Securities Exchange Act of 1934, as amended (the “Exchange Act”), the information set forth in Annex A is being provided to the Commission on a confidential supplemental basis only and is not to be filed with or deemed part of
the Exchange Act filings or any other filing made by the Company. The Company respectfully requests that the Staff return the unredacted version of this letter to it pursuant to Rule 24b-2 of the Exchange Act once the Staff has completed its review.
We have provided a self-addressed stamped envelope for this purpose. Kindly acknowledge receipt of this letter by stamping the enclosed copy of this letter and returning it in the envelope provided.

Should you have any further comments or questions with regard to the foregoing, please do not hesitate to contact the undersigned by phone at
781-419-1862 or by e-mail at jsnodgres@repligen.com.

Sincerely,

 /s/ Jon Snodgres

Name: Jon Snodgres

Title: Chief Financial Officer

Cc:

 Tony J. Hunt

             Repligen Corporation

 Arthur R. McGivern

 Courtney M. Gaughan

             Goodwin Procter LLP

 Securities and Exchange Commission

January 24, 2017

  Page
 5

 ANNEX A

(in thousands)

Nine Months
ended
September 30,

Twelve Months ended December 31,

2016

2015

2014

 Protein A Ligands

$
[***]

$
[***]

$
[***]

 Protein A Chromatography Resins

[***]

[***]

[***]

 Growth Factors (incl. Long® R3
IGF-1)

[***]

[***]

[***]

 OPUS® Prepacked Chromatography Columns
(1)

[***]

[***]

[***]

 ATF Systems and Consumables (2)

[***]

[***]

[***]

 Other Products

[***]

[***]

[***]

 Total Product Revenue

$
[***]

$
[***]

$
[***]

(1)
[***]

(2)
[***]

CORRESP
1
filename1.htm

CORRESP

 January 9, 2017

Via EDGAR

 Securities and Exchange Commission

 Division of Corporation Finance

 100 F Street, N.E.

Washington, DC 20549-3720

RE:
Repligen Corporation

 Form 10-K for year ended December 31, 2015

Filed February 25, 2016

File No. 0-14656

 Ladies and Gentlemen:

 Repligen Corporation (the “Company”) hereby advises the staff (the “Staff”) of the Securities and
Exchange Commission (the “Commission”) that the Company has received the Staff’s letter dated December 22, 2016 (the “Comment Letter”), regarding the Commission’s review of the Company’s Annual
Report on Form 10-K for the fiscal year ended December 31, 2015. The Comment Letter requests that the Company respond within ten (10) business days from the date thereof, or inform the Staff of when the Company would provide a response.
The Company respectfully requests an extension of an additional ten (10) business days until January 24, 2017 to respond to the Comment Letter in order to provide sufficient time for the Company to prepare the necessary response. The
Company is committed to responding to the Comment Letter promptly and intends to provide a response to the Staff no later than January 24, 2017. Should you have any questions regarding the request made herein, please do not hesitate to contact
me at (781) 419-1862. Thank you very much for your courtesy and cooperation in this matter.

Sincerely,

 /s/ Jon Snodgres

Jon Snodgres

Chief Financial Officer

cc:
Arthur R. McGivern – Goodwin Procter LLP

Mail Stop 4546
December 22, 2016

Via E -mail
Mr. Jon Snodgres
Chief Financial Officer
Repligen Corporation
41 Seyon Street
Bldg. 1, Suite 100
Waltham, MA  0245 3

Re: Repligen Corporation
 Form 10-K for the fiscal year ended December 31, 2015
Filed February  25, 2016
File No. 0 -14656

Dear Mr. Snodgres :

We have reviewed your filing an d have the following comment.  In our comment, we ask
you to provide us with information so we may better understand your disclosure.

Please respond to this comment  within ten busine ss days by providing the requested
information or advis e us as soon as possible when you will respond.  If you do not believe our
comment appl ies to your facts and circumstances, please tell us why in your response.

After reviewing your response and any amendment you may file in response to th is
comment , we may have  additional comments.

Financial Statements
Segment Reporting, page 65

1. Please p rovide us, for the fiscal years 2015 and 2014 and for the nine months ended
September 30, 2016, product revenues for :
 Protein A ligands ;
 OPUS® pre -packed chromatography column line ;
 Protein A chromatography resins ;
 LONG®R3 IGF -1; and
 ATF System
as discuss ed under “Our Products” on page 3.  Also, te ll us your consideration for
providing product revenue disclosures pursuant to  ASC 280 -10-50-40.

Mr. Jon Snodgres
Repligen Corporation
December 22, 2016
Page 2

 We remind you that the company and its management are responsible for the accuracy
and adequacy of their disclosur es, notwithstanding any review, comments, action or absence of
action by the staff.

You may contact Lisa Vanjoske, Assistant Chief Accountant  at (202) 551 -3614  or Mary
Mast, Senior Accountant  at (202) 551 -3613  if you have questions regarding comments on  the
financial statements and related matters.   In this regard, do not hesitate to contact me at (202)
551-3679 .

Sincerely,

 /s/ Jim B. Rosenberg

Jim B. Rosenberg
Senior Assistant Chief Accountant
Office of Healthcare and Insurance

February 25, 2015

Via E -mail
Mr. Jon Snodgres
Chief Financial Officer
Repligen Corporation
41 Seyon Street, Building 1, Suite 100
Waltham, MA 82453

Re: Repligen Corporation
 Form 10-K for Fiscal Year Ended December 31, 2013
Filed March 14, 2014
File No. 000 -14656

Dear Mr. Snodgres :

We have completed our review of your filing .  We remind you that our comments or
changes to disclosure in response to our comments do not foreclose the Commission from taking
any action with respect to the company or the filing and the company may not assert staff
comments as a defense in any proceeding initiated by the Commission or any person under the
federal securities laws of the United States.  We u rge all persons who are responsible for the
accuracy and adequacy of the disclosure in the filing to be certain that the filing include s the
information the Securities Exchange Act of 1934 and all applicable rules require.

Sincerely,

 /s/ Jeffrey P. Riedler

 Jeffrey P. Ried ler
Assistant Director

CORRESP
1
filename1.htm

Correspondence

 February 19, 2015

Via EDGAR

 Jeffrey Riedler

Assistant Director

 Division of Corporation Finance

U.S. Securities and Exchange Commission

 100 F Street, N.E.

Washington, DC 20549

Re:
Repligen Corporation

 Form 10-K for the fiscal year ended December 31, 2013

Filed March 14, 2014

File No. 000-14656

 Dear
Mr. Riedler:

 This letter is submitted on behalf of Repligen Corporation (the “Company”) in response to comments of
the staff of the Division of Corporation Finance (the “Staff”) of the Securities and Exchange Commission (the “Commission”) with respect to the Company’s Form 10-K for the fiscal year ended December 31,
2013 (the “Form 10-K”) as set forth in your letter, dated February 13, 2015, and prior letter dated December 23, 2014.

For reference purposes, the Staff’s numbered comments have been reproduced in italics herein with responses immediately following such
comment. Page references in the responses refer to the pages in the Form 10-K as filed with the Commission.

 Research and Development, page 4

1. We note your response to our prior comment 2. Notwithstanding your confidential treatment for provisions and terms in the HDAC Agreement
with BioMarin, we deem information regarding the parties’ rights and obligations, duration of the agreement and termination provisions to be material to investors. Accordingly, please expand your proposed disclosure to be included in future
filing to provide the parties rights and obligations, duration of the agreement and termination provisions.

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 February 19, 2015

 Page
 2

 Response to Comment No. 1:

The Company acknowledges the Staff’s comment. In future filings, the Company will expand its disclosure regarding the HDAC Agreement with
BioMarin substantially as set forth below in italicized and underlined text.

 HDAC Agreement with BioMarin

On January 21, 2014, we out-licensed our HDACi portfolio, which includes the Friedreich’s ataxia program, to BioMarin
Pharmaceuticals Inc. Friedreich’s ataxia is an inherited disease that causes progressive damage to the nervous system resulting in symptoms ranging from impaired walking and speech problems to heart disease. Pursuant to the terms of the
agreement, BioMarin agrees to use commercially reasonable efforts to commercialize HDACi portfolio product until the later of (i) the expiration of the last-to-expire valid claim of an issued and unexpired patent or pending patent application
claiming a compound included in the agreement or (ii) 10 years. Under the terms of the agreement, Repligen received an upfront payment of $2 million in January 2014 from BioMarin and we have the potential to receive up to $160 million
in future milestone payments for BioMarin’s development, regulatory approval and commercial sale of portfolio compounds included in the agreement. These potential milestone payments are approximately 37% related to clinical
development and 63% related to initial commercial sales in specific geographies. In addition, Repligen is eligible to receive royalties on sales of qualified products developed. The royalty rates are tiered and begin in the mid-single-digits for the
first HDACi portfolio product and for the first non-HDACi portfolio product with lesser amounts for any backup products developed under the agreement. Repligen’s receipt of these royalties is subject to customary offsets and deductions. There
are no refund provisions in this agreement. Royalties under this agreement are paid on a country-by-country basis during the period beginning on the first commercial sale of a compound in such country, until the later of: (i) the
expiration of exclusivity period granted by a governmental authority to prevent the entry of generic product into such country; (ii) the expiration of the last-to-expire valid claim of an issued and unexpired patent or pending patent
application claiming such compound in such country; or (iii) ten years following the first commercial sale of such HDACi portfolio product in any country. Royalty payments on products derived from the compounds included in the agreement are
calculated by multiplying net sales of such product for the calendar year by an applicable royalty rate based on incremental net sale amounts.

2. We note your response to our prior comment 3. Notwithstanding your confidential treatment request for cessation provisions of
royalties, we deem information relating to the duration of the agreement to be material to investors. Accordingly, please expand your proposed disclosure to be included in future filings to provide the expiration date of the last to expire patent
and the “predetermined time” referenced in your disclosure on which the payment of royalties is based.

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 February 19, 2015

 Page
 3

 Response to Comment No. 2:

The Company acknowledges the Staff’s comment and respectfully directs the Staff’s attention to the Company’s Current Report on
Form 8-K, filed on January 30, 2015, in which the Company discloses that, on January 26, 2015, the Company received from Pfizer a notice of termination for convenience under the SMA Agreement. In light of the termination of the SMA
Agreement, in future filings, the Company will modify its disclosure regarding the SMA Agreement with Pfizer substantially as set forth below in italicized and underlined text.

SMA Agreement with Pfizer

 On
December 28, 2012, we entered into an exclusive worldwide licensing agreement (the “License Agreement”) with Pfizer to advance the SMA program, which is led by RG3039 and also includes backup compounds and enabling technologies. Under
the terms of the License Agreement, we received $5 million from Pfizer as an upfront payment on January 22, 2013 and a $1 million milestone payment on September 4, 2013. Pursuant to this License Agreement, Pfizer assumed full
responsibility for the SMA program, including the conduct of the clinical trials necessary for any product approvals. On January 26, 2015, Pfizer issued to us a notice of its termination of the License Agreement for convenience, effective
as of April 26, 2015.

 Patent, Licenses and Proprietary Rights, page 6

3. We note your response to our prior comment 5. Please expand your proposed disclosure to provide the expected expiration date if
your HDAC inhibitors patent application is granted.

 Response to Comment No. 3:

The Company acknowledges the Staff’s comment. In future filings, the Company will expand its disclosure regarding its HDAC patent
applications substantially as set forth below in italicized and underlined text:

 On January 21, 2014, the Company
out-licensed its HDAC Inhibitor (HDACi) portfolio to BioMarin Pharmaceuticals Inc. The Company’s outlicensed HDACi portfolio included patent applications in the United States as well as patent applications in the EU, Canada, Japan and
Australia. Patents, if any, that are granted based on these patent applications are expected to expire from 2029 to 2032.

*    *    *

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 February 19, 2015

 Page
 4

 On behalf of the Company, I hereby acknowledge that:

•

the Company is responsible for the adequacy and accuracy of the disclosure in the filing;

•

Staff comments or changes to disclosure in response to Staff comments do not foreclose the Commission from taking any action with respect to the filing; and

•

the Company may not assert Staff comments as a defense in any proceeding initiated by the Commission or any person under the federal securities laws of the United States.

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 February 19, 2015

 Page
 5

 Should you have any further comments or questions with regard to the foregoing, please do not
hesitate to contact the undersigned by phone at 781-419-1862 or by e-mail at jsnodgres@repligen.com or Arthur R. McGivern by phone at 617-642-5702 or by email at amcgivern@goodwinprocter.com.

Sincerely,

 /s/ Jon Snodgres

Name:

Jon Snodgres

Title:

Chief Financial Officer

Cc:
Walter C. Herlihy

 Repligen Corporation

Arthur R. McGivern

 Courtney M.
Gaughan

 Goodwin Procter LLP

February 13, 2015

Via E -mail
Mr. Jon Snodgres
Chief Financial Officer
Repligen Corporation
41 Seyon Street, Building 1, Suite 100
Waltham, MA 82453

Re: Repligen Corporation
 Form 10-K for Fiscal Year Ended December 31, 2013
Filed March 14, 2013
File No. 000 -14656

Dear Mr. Snodgres :

We have reviewed your respo nse letter dated January 20, 2015  and have the following
comments.  Please respond to this letter within ten busines s days by  amending your filing  or by
advising us when you will provide the requested response.   If you do not believe our comment s
apply  to your facts and circumstances or do not believe an amendment is appropriate, please tell
us why in your response.

After rev iewing any amendment to your filing and the information you provide in
response to these  comment s, we may have  additional comments.

Research and Development, page 4

1. We note your response to our prior comment 2.  Notwithstanding your confiden tial
treatment request for provisions and terms in the HDAC Agreement with BioMarin, we
deem information regarding the parties’ rights and obligations, duration of the agreement
and termination provisions to be material to investors.  Accordingly, please e xpand your
proposed disclosure to be included in future filing to provide the parties rights and
obligations, duration of the agreement and termination provisions.

2. We note your response to our prior comment 3.  Notwithstanding your confidential
treatmen t request for cessation provisions of royalties, we deem information relating to
the duration of the agreement to be material to investors.  Accordingly, please expand
your proposed disclosure to be included in future filings to provide the expiration date  of
the last to expire patent and the “predetermined time” referenced in your disclosure on
which the payment of royalties is based.

Mr. Jon Snodgres
Repligen Corporation
February 13, 2015
Page 2

 Patent, License and Proprietary Rights, page 6

3. We note your response to our prior comment 5.  Please expand your prop osed disclosure
to provide the expected expiration date if your HDAC inhibitors patent application is
granted.

You may contact Johnny Gharib at  (202) 551 -3170, John Krug  at (202) 551 -3862  or me
at (202) 551 -3715  with any questions.

Sincerely,

 /s/ Bryan J. Pitko for

 Jeffrey P. Riedler
Assistant Director

CORRESP
1
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Correspondence

 January 20, 2015

Via EDGAR

 Jeffrey Riedler

Assistant Director

 Division of Corporation Finance

U.S. Securities and Exchange Commission

 100 F Street, N.E.

Washington, DC 20549

Re:
Repligen Corporation

 Form 10-K for the fiscal year ended December 31, 2013

 Filed March 14, 2014

File No. 000-14656

 Dear
Mr. Riedler:

 This letter is submitted on behalf of Repligen Corporation (the “Company”) in response to comments of
the staff of the Division of Corporation Finance (the “Staff”) of the Securities and Exchange Commission (the “Commission”) with respect to the Company’s Form 10-K for the fiscal year ended December 31,
2013 (the “Form 10-K”) as set forth in your letter, dated December 23, 2014 (the “Comment Letter”).

For reference purposes, the Staff’s numbered comments have been reproduced in italics herein with responses immediately following such
comment. Page references in the responses refer to the pages in the Form 10-K as filed with the Commission.

 Our Products for the Manufacturing of
Biologic Drugs, page 2

 1. We note that your Strategic Supplier Alliance Agreement with GE Healthcare appears to be a
material agreement as it has been filed as an exhibit pursuant to Item 601(b)(10) of Regulation S-K. Please expand your disclosure regarding the agreement to provide the material terms, including the parties’ rights and obligations, the
duration of the agreement, termination provisions, and any payment provisions.

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 January 20, 2015

 Page
 2

 Response to Comment No. 1:

The Company acknowledges the Staff’s comment. In future filings, the Company will expand its disclosure regarding the GE Healthcare
Strategic Supplier Alliance Agreement substantially as set forth below in italicized and underlined text, which disclosure the Company respectfully notes was included in its Current Report on Form 8-K announcing its entry into the agreement. The
Company respectfully directs the Staff’s attention to the application for confidential treatment, dated February 12, 1999, and the application for continued confidential treatment, dated December 16, 2008, in which the Company
petitioned the Staff for confidential treatment for, in addition to other things, certain payment provisions and terms under the GE Healthcare Strategic Supplier Alliance Agreement.

Our Products for the Manufacturing of Biologic Drugs

Repligen is a leading developer and manufacturer of certain consumable bioprocessing products used in the production of monoclonal antibodies
and other biologic drugs. The Company manufactures multiple forms of Protein A ligand, a critical component of Protein A media that is used in the downstream purification process for monoclonal antibodies, on behalf of several major life sciences
companies. We also manufacture and sell growth factors, used to increase cell growth and productivity during upstream fermentation, and chromatography products. Our chromatography products include OPUS pre-packed columns for biologics purification,
proprietary Protein A media and quality test kits. These products are sold to life sciences companies, contract manufacturing organizations and biopharmaceutical companies for use in the biologic drug production. Demand for our bioprocessing
products has grown in concert with the expanding global market for biologics, particularly monoclonal antibodies, and also as a result of new product offerings through our acquisition of the Novozymes Biopharma business in December 2011.

In 2012, the global biologics market was valued at approximately $175 billion and is expected to grow at a rate in the high single digits
annually. Market research indicates that the monoclonal antibody segment comprised over 40% of the overall biologics market in 2012 and is growing more rapidly than the overall market. Six of the ten worldwide best-selling drugs in 2012 were
monoclonal antibodies, including blockbuster products such as Enbrel® and Remicade® for the treatment of rheumatoid arthritis and other
inflammatory disorders, Rituxan® for non-Hodgkin’s lymphoma and Herceptin® for the treatment of breast cancer. There are more than
35 approved monoclonal antibody products and over 350 product candidates currently in clinical development, most of which are manufactured using Protein A.

Repligen has been a leading manufacturer of Protein A for over fifteen years and manufactures multiple forms of Protein A for major
life sciences companies including GE Healthcare and EMD Millipore under long-term supply agreements which extend to dates between 2016 and 2021. Pursuant to Repligen’s Strategic Supplier Alliance Agreement with GE Healthcare, Repligen has
agreed to manufacture and sell to GE Healthcare two forms of GE Healthcare proprietary recombinant Protein A through January 28, 2015, with a right to extend the agreement through 2021. Repligen’s Strategic Supplier Alliance Agreement with
GE Healthcare is terminable by either party upon written notice of breach or insolvency. In addition,

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 January 20, 2015

 Page
 3

pursuant to the Strategic Supplier Alliance Agreement, or Sweden Supplier Agreement, by and between Repligen Sweden AB (as successor-in-interest to Novozymes Biopharma Sweden AB) and GE
Healthcare, Repligen agrees to manufacture and sell to GE Healthcare three forms of GE Healthcare proprietary recombinant Protein A through December 31, 2016. Either party may terminate the Sweden Supplier Agreement unilaterally for convenience
upon two years’ written notice or upon written notice of material default or breach of the agreement and bankruptcy or insolvency . GE Healthcare may terminate the Sweden Supplier Agreement if Repligen markets or sells any chromatography media
products, transfers control of the Sweden Supplier Agreement or upon delay or defect in the proprietary recombinant Protein A products. To be useful in the monoclonal antibody manufacturing process, Protein A is chemically bound to
proprietary microscopic beads that are manufactured by life sciences companies, such as those mentioned above. These beads provide the rigid support required to use Protein A ligands. The combination of Protein A ligands bound to the beads is known
as Protein A chromatographic media, which is packed by end-users into cylindrical columns and used to purify monoclonal antibodies. For example, after a fermentation process that produces monoclonal antibodies, the broth containing the monoclonal of
interest, as well as numerous fermentation by-products and contaminants, is pumped through a column filled with Protein A chromatographic media. The Protein A media selectively binds to or “captures” the monoclonal antibody. Protein A has
a high affinity for the monoclonal antibody and as a result, the antibody remains bound to the Protein A media while impurities flow through the column and are discarded. Once the impurities are removed, a change in pH conditions releases the
purified antibody from the Protein A media. As a result, the monoclonal antibody product is highly purified and concentrated from a single purification step. Further purification steps are usually necessary to increase purity to a level greater than
98%. Over the past three years, the majority of our product sales have been sales of Protein A products.

 Most biopharmaceuticals
are produced through mammalian cell fermentation. In order to spur increased cell growth, manufacturers add growth factors and nutrients to the fermentor. As part of the Novozymes Acquisition, the Company acquired four cell culture growth factor
products. Among those products is LONG®R3 IGF-I, a growth factor that is more biologically potent than insulin, and that has been shown to significantly increase recombinant protein production
in fermentation applications. LONG®R3 IGF-I is currently used in the manufacture of several commercial biopharmaceutical products and is sold under a distribution agreement with Sigma-Aldrich
Corporation (“Sigma”) which extends to 2021. Sigma has distribution rights for industrial cell culture applications while Repligen sells the product for use in stem cell and other cell-based therapies. In addition, we acquired long
epidermal growth factor (LONG®EGF) and transforming growth factor alpha (LONG®TGF-a) supplements for serum-free or low serum culture in
cell-based therapy applications, as well as recombinant transferrin (rTransferrin) which has been developed as an iron supplement for cell culture. There may be applications for these growth factors in stem cell and other cell-based therapies.

We also sell a number of products used in purification and quality control applications to contract manufacturers and biopharmaceutical
companies. These products include: OPUS pre-packed, disposable chromatography columns, proprietary Protein A chromatography media and

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 January 20, 2015

 Page
 4

quality test kits. Our pre-packed chromatography columns are sold in a variety of sizes with the customer’s choice of media. This product line’s smaller sizes consist of proprietary
technology that we acquired from BioFlash Partners, LLC (“BioFlash”) in January 2010 while the larger sizes encompass products and technology that we developed as a result of our internal research and development efforts. The OPUS brand
stands for “Open Platform, User Specified.” OPUS columns have the potential to improve manufacturing efficiencies and lower costs by reducing labor and time spent on column packing, validation, set-up and cleaning. In addition, because
OPUS columns are “plug-and-play” we believe they offer customers significantly greater manufacturing efficiency and flexibility when used with other flexible, disposable technologies. In early 2012, we introduced new, process-scale OPUS
chromatography columns with diameters of 20cm and 30cm. These new products are well suited for the production of a broad range of clinical trial material and niche commercial products such as orphan biologics.

Our proprietary Protein A chromatography media is used by contract manufacturers and biopharmaceutical companies in a variety of applications,
including in the purification of some currently marketed biotherapeutics. Customers use our Protein A and Growth Factor ELISA test kits to ensure that there are minimal levels of residual Protein A and growth factor, respectively, in the final bulk
drug product.

 Research and Development, page 4

2. Please expand your disclosure for your HDAC Agreement with BioMarin to describe the material terms of the agreement, including the
parties’ rights and obligations, the duration of the agreement, termination provisions and royalty rates. Also, please file the agreement as an exhibit pursuant to Item 601(b)(10) of Regulation S-K.

Response to Comment No. 2:

The Company acknowledges the Staff’s comment. In future filings, the Company will expand its disclosure regarding the HDAC Agreement with
BioMarin substantially as set forth below in italicized and underlined text. The Company respectfully informs the Staff that because the HDAC Agreement with BioMarin was entered into during the quarter ended March 31, 2014, it is filed as
Exhibit 10.01 to the Company’s quarterly report on form 10-Q for the quarter ended March 31, 2014, filed with the Commission on May 9, 2014. The Company respectfully directs the Staff’s attention to the application for
confidential treatment, dated May 9, 2014, in which the Company petitioned the Staff for confidential treatment for, in addition to other things, certain provisions and terms under the HDAC Agreement with BioMarin.

HDAC Agreement with BioMarin

 On
January 21, 2014, we out-licensed our HDACi portfolio, which includes the Friedreich’s ataxia program, to BioMarin Pharmaceuticals Inc. Friedreich’s ataxia is an inherited

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 January 20, 2015

 Page
 5

disease that causes progressive damage to the nervous system resulting in symptoms ranging from impaired walking and speech problems to heart disease. Under the terms of the agreement, Repligen
received an upfront payment of $2 million in January 2014 from BioMarin and we have the potential to receive up to $160 million in future milestone payments for the development, regulatory approval and commercial sale of portfolio compounds included
in the agreement. These potential milestone payments are approximately 37% related to clinical development and 63% related to initial commercial sales in specific geographies. In addition, Repligen is eligible to receive royalties on
sales of qualified products developed. The royalty rates are tiered and begin in the mid-single-digits for the first HDACi portfolio product and for the first non-HDACi portfolio product with lesser amounts for any backup products developed
under the agreement. Repligen’s receipt of these royalties is subject to customary offsets and deductions. There are no refund provisions in this agreement.

3. Please expand your disclosure for your SMA Agreement with Pfizer to describe the termination provisions of the agreement. With
regard to the duration of the agreement, we note that the royalty term of the agreement is based on the later of (a) expiration of the licensed patents or (b) a predetermined time after the first commercial sale of the first product in a
country. Please revise the duration disclosure to provide the expiration date of the last to expire patent and the “predetermined time” referenced in your disclosure.

Response to Comment No. 3:

The Company acknowledges the Staff’s comment. In future filings, the Company will expand its disclosure regarding the SMA Agreement with
Pfizer substantially as set forth below in italicized and underlined text. The Company respectfully directs the Staff’s attention to the application for confidential treatment, dated March 15, 2013, as amended on May 2, 2013, in which
the Company petitioned the Staff for confidential treatment for, in addition to other things, certain royalty adjustment and cessation provisions and terms under the Pfizer Inc. License Agreement.

SMA Agreement with Pfizer

 On
December 28, 2012, we entered into an exclusive worldwide licensing agreement (the “License Agreement”) with Pfizer to advance the SMA program, which is led by RG3039 and also includes backup compounds and enabling technologies. Under
the terms of the License Agreement, we received $5 million from Pfizer as an upfront payment on January 22, 2013 and a $1 million milestone payment on September 4, 2013. We are entitled to receive up to $64 million in potential future
milestone payments, a portion of which may be owed to third parties. These potential payments are approximately equally divided between milestones related to clinical development and initial commercial sales in specific geographies. In
addition, we are entitled to receive royalties on any future sales of RG3039 or any SMA compounds developed under the License Agreement. The License Agreement also provides for tiered and increasing royalty rates which begin in the high
single-digits for RG-3039 or lesser amounts for any backup compounds

 Mr. Jeffrey Riedler

Securities and Exchange Commission

 January 20, 2015

 Page
 6

developed under the License Agreement. Our receipt of these royalties is subject to an obligation under an existing in-license agreement and other customary offsets and deductions.
Royalties are payable, on a country-by-country basis, for a duration based upon the later of (a) expiration of the licensed patent(s) and (b) a predetermined time after the first commercial sale of the first such product in
such country. Pursuant to this License Agreement, Pfizer has assumed full responsibility for the SMA program moving forward, including the conduct of the clinical trials necessary for any p

CORRESP
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SEC Letter

 December 30, 2014

Via EDGAR

 Securities and Exchange Commission

 100 F Street, N.E.

 Washington, DC 20549

Attention: Jeffrey P. Riedler, Assistant Director

RE:
Repligen Corporation

Form 10-K for year ended December 31, 2013

Filed March 14, 2014

File No. 000-14656

 Ladies and Gentlemen:

Repligen Corporation (the “Company”) hereby advises the staff (the “Staff”) of the Securities and Exchange
Commission (the “Commission”) that the Company has received the Staff’s letter dated December 23, 2014 (the “Comment Letter”), regarding the Commission’s review of the Company’s Annual Report on
Form 10-K for the fiscal year ended December 31, 2013. The Comment Letter requests that the Company respond within ten (10) business days from the date thereof, or inform the Staff of when the Company would provide a response. The Company
respectfully requests an extension of an additional ten (10) business days until January 23, 2015 to respond to the Comment Letter in order to provide sufficient time for the Company to prepare the necessary response. The Company is
committed to responding to the Comment Letter promptly and intends to provide a response to the Staff no later than January 23, 2015. Should you have any questions regarding the request made herein, please do not hesitate to contact me at
(781) 419-1862. Thank you very much for your courtesy and cooperation in this matter.

Sincerely,

/s/ Jon Snodgres

 Jon Snodgres

 Chief Financial
Officer

cc:
Arthur R. McGivern – Goodwin Procter LLP

December 23, 2014
Via E -mail
Mr. Jon Snodgres
Chief Financial Officer
Repligen Corporation
41 Seyon Street, Building 1, Suite 100
Waltham, MA 82453

Re: Repligen Corporation
 Form 10-K for the year ended December 31, 2013
Filed March 14, 2014
File No. 000 -14656

Dear Mr. Snodgres:

We have reviewed your filing and have the following comments.  In some of our
comments, we may ask you to provide us with information so we may better unders tand your
disclosure.

Please respond to this letter within ten business days by amending your filing, by
providing the requested information, or by advising us when you will provide the requested
response.   If you do not believe our comments apply to your facts and circumstance s or do
not believe an amendment is appropriate, please tell us why in your response.

After reviewing any amendment to your filing and the information you provide in
response to these comments, we may have additional comments.

Our Products for the Manufacturing of Biologic Drugs, page 2

1. We note that your Strategic Supplier Alliance Agreement with GE Healthcare appears
to be a material agreement as it has been filed as an exhibit pursuant to Item
601(b)(10) of Regulation S -K.  Please expand your disclosure regarding the
agreement to provide the material terms, including the parties’ rights and obligations,
the duration of the agreement, termination provisions, and any payment provisions.

Research and Develop ment, page 4

2. Please expand your disclosure for your HDAC Agreement with BioMarin to describe
the material terms of the agreement, including the parties’ rights and obligations, the
duration of the agreement, termination provisions and royalty rates.  Also , please file
the agreement as an exhibit pursuant to Item 601(b)(10) of Regulation S -K.

3. Please expand your disclosure for your SMA Agreement with Pfizer to describe the
termination provisi ons of the agreement.  With regard to the duration of the
agreement, we note that the royalty term of the agreement is based on the later of (a)

Mr. Jon Snodgres
Repligen Corporation
December 23, 2014
Page  2

 expiration of the licensed patents or (b) a predetermined time after the first
commercial sale of the first pro duct in a country.  Please revise the duration
disclosure to provide the expiration date of the last to expire patent and the
“predetermined time” referenced in your disclosure.

Significant Customers and Geographic Reporting, page 5

4. Please revise your di sclosure in this section to identify the customer which accounted
for 35% of your revenues for the fiscal year ended December 31, 2013.  Also, please
identify customers A, B and C in the section entitled “Concentrations of Credit Risk
and Significant Custo mers” on page 67.

Patent, Licenses and Proprietary Rights, page 6

5. Please expand your disclosure for your material patents and patent applications to
include the following information:

 the type of patent protection such as composition of matter, use or process for
your U.S. Patent No. 7,691,608 B2;
 the jurisdiction where your OPUS pre -packed column patent application is
pending and the expected expiration date if the patent is issued;
 the expiration dates of U.S. Patent Nos. 7,888,366 and 7,985,755; and
 the jurisdiction where your histone deacetylase inhibitors patent application is
pending and the expected expiration date if the patent is issued.

We urge all persons who are responsible for the accuracy and adequacy of the
disclosure in the filing to be certain that the filing includes the information the Securities
Exchange Act of 1934 and all applicable Exchange Act rules require.   Since the compa ny and
its management are in possession of all facts relating to a company’s disclosure, they are
responsible for the accuracy and adequacy of the disclosures they have made.

 In responding to our comments, please provide a written statement from the co mpany
acknowledging that:

 the company is responsible for the adequacy and accuracy of the disclosure in the
filing;
 staff comments or changes to disclosure in response to staff comments do not
foreclose the Commission from taking any action with respect to the filing; and
 the company may not assert staff comments as a defense in any proceeding initiated
by the Commission or any person under the federal securities laws of the United
States.

You may contact Johnny Gharib  at (202) 551 -3170, John Krug at (202) 551 -3862 or
me at (202) 551 -3715 with any questions.

Mr. Jon Snodgres
Repligen Corporation
December 23, 2014
Page  3

Sincerely,

 /s/ Jeffrey Riedler

 Jeffrey Riedler
Assistant Director

Via Facsimile and U.S. Mail
Mail Stop 4720
                                                                                                 September 10, 2009  Walter C. Herlihy, Ph.D. President, Chief Executive Officer and Director 41 Seyon Street, Building #1, Suite 100 Waltham, Massachusetts 02453
Re: Repligen Corporation
  Form 10-K for the Year Ended March 31, 2008   Filed June 13, 2008   DEF 14A filed July 29, 2008   File No. 000-14656

Dear Dr. Herlihy:
  We have completed our review of your Form 10-K and related filings and have
no further comments at this time.

Sincerely,

Jeffrey P. Riedler Assistant Director

CORRESP
1
filename1.htm

Correspondence

 Repligen Corporation

 41 Seyon Street, Building #1, Suite 100

 Waltham, Massachusetts 02453

 September 9, 2009

 United States Securities and Exchange
Commission

 Division of Corporation Finance

 100 F Street N.E.

 Washington, D.C. 20549

 Attention: Jeffrey Riedler, Assistant
Director

RE:
Repligen Corporation

 Supplemental Response to Form 10-K
for the Year Ended March 31, 2008

 Filed March 31, 2009

 File No. 000-14656

 Dear Mr. Riedler:

 This letter is being furnished in response to the comments of the Staff (the “Staff”) of the Securities and Exchange Commission (the
“Commission”) in the Commission’s letter dated July 14, 2009 (the “Comment Letter”) to Dr. Walter C. Herlihy, Ph.D., President, Chief Executive Officer and Director of Repligen Corporation
(“Repligen” or the “Company”), with respect to the Company’s Supplemental Response to Form 10-K for the fiscal year ended March 31, 2008, filed May 29, 2009. The response set forth below has been organized in the
same manner in which the Commission’s comments and headings were organized in the Comment Letter.

 Form l0-K for year ended March 31, 2008

 Item 1. Business, page 3

1.
Please provide us with an analysis supporting your determination that you were not substantially dependent on your agreement with Applied as of March 31, 2008. Your analysis
should focus on the facts and circumstances as of March 31, 2008. If you had reason to believe that your dependence of this agreement would decline after March 31, 2008, this information may be relevant but would not be determinative.

 RESPONSE: We respectfully advise the Staff that our rationale for determining that Repligen Corporation was not materially
dependent upon Applied as of March 31, 2008 is based on an assessment of:

•

 the nature of our business as a development stage research and development biotechnology company and how these revenues are viewed internally in relation to the
long term success of the organization;

•

 the materiality of the relationship compared to our overall operations at March 31, 2008;

•

 the potential impact a disruption of that relationship would have on our results and financial position.

 Most development stage biotechnology companies do not have an identifiable revenue source, but rather rely to a great extent on the capital markets to
fund operations as they advance their clinical products to the market. This was especially the case as of March 31, 2008 prior to the credit crisis. Repligen is no different than our peers in this regard and has historically accessed the
capital markets for such funding. Fortuitously, the Company historically has also had product line revenue which partially funds operations and Applied is a customer in this regard. Further, as discussed below, our intellectual property assets have
become a much greater source of funding as well. Our core focus, however, remains our research and development operations, and even a complete deterioration in revenue from Applied would not materially detract the Company from continuing to pursue
those goals as we would follow the traditional development stage biotechnology model of accessing the capital markets.

 Further, during the
fiscal year ended March 31, 2008, the Company, as disclosed in Note 10 in our Form 10-K, successfully entered into a Settlement Agreement with ImClone Systems, Inc. that yielded $40,170,000 in net proceeds for the Company. The proceeds from
this settlement agreement, in addition to the $18,587,000 of revenue, totaled $58,757,000 of funding from operations for the Company in fiscal 2008. Applied, therefore, represented less than 5% of that funding. Conversely, our largest customer, GE
Healthcare, still represented nearly 20% of this total. In addition, the company announced a significant settlement agreement with Bristol-Myers Squibb on April 7, 2008, as disclosed in Note 12 in our Form 10-K, that we believe will result in
cash flows far in excess of the ImClone settlement over the coming years. As a result, and in recognition of the fact that we were seeing Applied revenues trend downward during this timeframe, we did not consider the Company to be financially
dependent upon Applied as of March 31, 2008.

2.
We note your response to our prior comment 2. Please provide proposed disclosure to include information regarding the expiration of the agreement and each party’s
termination rights thereunder. We note that your Form 10-K for the fiscal year ended March 31, 2009 filed June 11, 2009 does not contain this information.

 2

 RESPONSE: We respectfully advise the Staff that we would propose to include in the Company’s annual
report on Form 10-K for the year ended March 31, 2010, the following disclosure regarding the material terms of the license agreement with the University of Michigan. All proposed changes to such disclosure compared to previously submitted
disclosures in our annual report on Form 10-K for the year ended March 31, 2009 to incorporate the Staff’s recommendations are highlighted in bold italic print:

 “CTLA4-Ig

 CTLA4
is a key regulator of the activity of the immune system. CTLA4 “turns off” the immune system after it has successfully cleared a bacterial or viral infection by blocking the activation of T-cells, the immune cells responsible for
initiating an immune response. In the 1990’s, our collaborators at the University of Michigan (“Michigan”) and the U.S. Navy demonstrated in animal models that a fusion protein consisting of fragments of CTLA4 and an
antibody (“CTLA4-Ig”) could be used to treat certain autoimmune diseases. This research finding resulted in the granting of U.S. patent No. 6,685,941 (“the ‘941 Patent”) covering the treatment of certain auto-immune
disorders including rheumatoid arthritis with CTLA4-Ig. The ‘941 Patent is owned by Michigan and exclusively licensed to Repligen until the expiration of the patent in February 2021. In consideration of this exclusive license,
Repligen agreed to pay Michigan 15% of all royalty income received, after deducting legal expenses. There are no annual or other fees associated with this agreement. Repligen is not currently, nor do we anticipate developing any products under
this agreement. As a result, the primary remaining obligations of Repligen under this agreement are to remit 15% of royalty income received to Michigan and to fund patent prosecution costs as they arise. This agreement may be terminated by either
party at any time, although Repligen’s obligation to remit 15% of royalty income received extends beyond any termination. Under this agreement, since its inception through fiscal year 2009, Repligen has paid $1.1 million to the
University of Michigan. CTLA4-Ig’s mechanism of action is different from the current therapies for autoimmune disease or organ transplant rejection, thus it may provide a treatment for patients who are refractory to existing therapies.”

 3

 ***

 The Company acknowledges that:

•

 the Company is responsible for the adequacy and accuracy of the disclosure in the filing;

•

 staff comments or changes to disclosure in response to staff comments do not foreclose the Commission from taking any action with respect to the filing; and

•

 the Company may not assert staff comments as a defense in any proceeding initiated by the Commission or any person under the federal securities laws of the United
States.

 If you have any questions with regard to the Company’s responses, need further supplemental
information or would like to discuss any of the matters covered in this letter, please contact the undersigned at (781) 250-0111.

Sincerely,

 /s/ William J. Kelly

William J. Kelly

Chief Financial Officer

 4

Mail Stop 4720                                                                                                   July 14, 2009   Walter C. Herlihy, Ph.D. President, Chief Executive Officer and Director 41 Seyon Street, Building #1, Suite 100 Waltham, Massachusetts 02453
Re: Repligen Corporation
  Supplemental Response to Form 10-K for the Year
Ended March 31, 2008
  Filed May 29, 2009   File No. 000-14656

Dear Dr. Herlihy:

We have reviewed your filing and have th e following comments.  In our comments,
we ask you to provide us with information to  better understand your disclosure.  Where it
requests you to revise disclosure, the information you provide should show us what the revised disclosure will look like and identify the annual or quarterly filing, as applicable, in
which you intend to first include  it.  If you do not believe th at revised disclosure is
necessary, explain the reason in your response.  After reviewing the information provided,
we may raise additional comments and/ or request that you amend your filing.
  Please understand that the purpose of our re view process is to assist you in your
compliance with the applicable disclosure requirements and to enhance the overall disclosure in your filing.  We look forward to  working with you in these respects.  We
welcome any questions you may have about our comments or on any other aspect of our review.  Feel free to call us at the telephone numbers listed at the end of this letter.

Form 10-K for the year ended March 31, 2008

Item 1. Business, page 3
1. Please provide us with an analysis supporting your determination that you were not substantially dependent on your agreemen t with Applied as of March 31, 2008.
Your analysis should focus on the facts and circumstances as of March 31, 2008.  If you had reason to believe that your depende nce of this agreement would decline
after March 31, 2008, this information ma y be relevant but would not be
determinative.
2. We note your response to our prior comment 2.  Please provide proposed disclosure

Walter C. Herlihy, Ph.D.
Repligen Corporation July 14, 2009 Page 2
 to include information regarding the expiration of the agreement and each party’s termination rights thereunder.  We note that your Form 10-K for the fiscal year
ended March 31, 2009 filed June 11, 2009 doe s not contain this information.

*    *    *    *

Please provide us the information requested within 10 business days or tell us when
you will provide us with a response.  Please furnish a cover letter with your response that keys your responses to our comments and provide any requested information.  Detailed cover letters greatly facilita te our review.  Please furnis h your letter on EDGAR under the
form type label CORRESP.

 We urge all persons who are responsible for the accuracy and adequacy of the
disclosure in the filing to be certain that the filing includes all in formation required under
the Securities Exchange Act of 1934 and that they have provided all information investors
require for an informed investment decision.  Since the company and its management are
in possession of all facts relating to a company’s disclosure, they are responsible for the accuracy and adequacy of the disclosures they have made.     In connection with responding to our co mments, please provide, in your letter, a
statement from the company acknowledging that:

• the company is responsible for the adequacy  and accuracy of the disclosure in the
filing;
• staff comments or changes to disclosure  in response to staff comments do not
foreclose the Commission from taking any action with respect to the filing; and
• the company may not assert staff comments as a defense in any proceeding initiated
by the Commission or any person under the federal securities laws of the United
States.

In addition, please be advise d that the Division of Enfo rcement has access to all
information you provide to the staff of the Divi sion of Corporation Fi nance in our review
of your filing or in response to our comment on your filing.
Please contact Laura Crotty, Staff Attorn ey, at (202) 551-3563 or Suzanne Hayes,
Legal Branch Chief, at (202) 551-3675 if you ha ve questions regardi ng the processing of
your response as well as any questions regarding co mments and related matters.
 Sincerely,

Jeffrey P. Riedler Assistant Director

CORRESP
1
filename1.htm

SEC Response Letter

 Repligen Corporation

 41 Seyon Street, Building #1, Suite 100

 Waltham, Massachusetts 02453

 May 29, 2009

 United States Securities and Exchange
Commission

 Division of Corporation Finance

 100 F Street N.E.

 Washington, D.C. 20549

 Attention: Jeffrey Riedler, Assistant
Director

RE:
Repligen Corporation

 Supplemental Response to Form 10-K
for the Year Ended March 31, 2008

 Filed March 31, 2009

 File No. 000-14656

 Dear Mr. Riedler:

 This letter is being furnished in response to the comments of the Staff (the “Staff”) of the Securities and Exchange Commission (the
“Commission”) in the Commission’s letter dated April 16, 2009 (the “Comment Letter”) to Dr. Walter C. Herlihy, Ph.D., President, Chief Executive Officer and Director of Repligen Corporation
(“Repligen” or the “Company”), with respect to the Company’s Supplemental Response to Form 10-K for the fiscal year ended March 31, 2008, filed March 31, 2009. The response set forth below has been organized in the
same manner in which the Commission’s comments and headings were organized in the Comment Letter.

 Form l0-K for year ended March 31, 2008

 Item 1. Business, page 3

1. a.
In your response to our prior comment 1 you state that Applied’s purchases of Protein A from the Company account for “less than 10% of the Company’s product
revenues and less than 4% of total revenues.” However, we note that your disclosure on page 5 of the 10-K under “Significant Customers” states that Applied was one of two customers whose purchases of Protein A products accounted for
more than 10% of the Company’s total revenues in 2008, 2007 and 2006. Please explain this apparent discrepancy.

    b.
 In addition, you state that the supply agreement does not obligate the parties to sell or purchase specific quantities of Protein A. Yet, you also state that
Repligen and Applied are in fact obligated with respect to the quantities of Protein A reflected in

Applied’s short-term forecasts of its Protein A needs and these amounts are not fixed. So, while amounts of Protein A under the supply agreement may
have been immaterial so far, it seems possible that they could become much larger in the future. If true, while the agreement might qualify as immaterial right now, depending on the revenues involved thus far, it could become material in the future.
Please inform us whether this is the case, and if so, that you will file the agreement as a material contract should revenues attributable to the contract become material.

 RESPONSE: We respectfully advise the Staff that the our response to the relative materiality of Applied was in consideration of our current and forecasted
sales to Applied in our fiscal year 2009 as of the time of our response letter. As a result, we did not believe that this was a material contract on a forward looking basis and apologize for any confusion. We agree that Applied has historically been
a significant customer in prior periods based upon their order volume and recognize that they may again be significant in the future.

 Our
primary rationale, however, for not filing this contract as an exhibit to our annual report on Form 10-K was that pursuant to Regulation S-K, Item 601(b)(10) and the subcategories therein, our agreement with Applied was made in the ordinary
course of business and the Company is not substantially dependent upon revenues from Applied. On a forward looking basis, Applied is expected to represent an even smaller percentage of our total revenue. Therefore we did not deem this a
“Material Contract” as defined. Furthermore, the agreement itself, were it to be disclosed, would not contain any specific quantity obligations that would make its relative materiality apparent. Quantity of demand is only outlined in the
periodic orders and forecasts from Applied that we receive in the ordinary course of business during the year. The agreement does state certain pricing to be utilized in the event of orders from Applied, but we would respectfully request that such
pricing terms be given confidential treatment to avoid unfair advantage being afforded to any potential competitors. As a result, disclosure of the supply agreement would not provide either quantity or price information and would be of limited
utility to a user of this information. To the extent that this agreement becomes a “material contract” as provided in Item 601(b)(10), such as if we become substantially dependent on sales of a major part of our products to Applied
pursuant to this agreement, we intend to file this agreement and seek confidential treatment for certain provisions of this agreement.

2.
We note your response to our prior comment 3. Please also provide the following material information regarding the exclusive license agreement between the company and the
University of Michigan:

a.
Total amounts received to date under the agreement;

b.
Any annual fees, if applicable;

c.
The number of additional products you anticipate you will develop under this agreement; and

d.
The term and termination provisions, including any payments the company would be required to make in the event of termination.

 2

 RESPONSE: We respectfully advise the Staff that other than the obligation to remit 15% of royalty
revenues received to the University of Michigan, there are no annual or other fees applicable to this agreement. There are no other significant obligations for either party and there are no material termination provisions or payments required upon
termination. Further, we do not anticipate developing any additional products under this agreement.

 Pursuant to your request, however, we
would further propose to include in the Company’s annual report on Form 10-K for the year ended March 31, 2009, the following disclosure regarding the material terms of the license agreement with the University of Michigan. All proposed
changes to such disclosure compared to previously submitted disclosures in our annual report on Form 10-K for the year ended March 31, 2008 to incorporate the Staff’s recommendations are highlighted in bold italic print:

 “CTLA4-Ig

 CTLA4
is a key regulator of the activity of the immune system. CTLA4 “turns off” the immune system after it has successfully cleared a bacterial or viral infection by blocking the activation of T-cells, the immune cells responsible for
initiating an immune response. In the 1990’s, our collaborators at the University of Michigan and the U.S. Navy demonstrated in animal models that a fusion protein consisting of fragments of CTLA4 and an antibody (“CTLA4-Ig”) could be
used to treat certain autoimmune diseases. This research finding resulted in the granting of U.S. patent No. 6,685,941 (“the ‘941 Patent”) covering the treatment of certain auto-immune disorders including rheumatoid arthritis
with CTLA4-Ig. The ‘941 Patent is owned by the University of Michigan and exclusively licensed to Repligen. In consideration of this exclusive license, Repligen agreed to pay the University of Michigan 15% of all royalty income received,
after deducting legal expenses. There are no annual or other fees associated with this agreement. Under this agreement, since its inception through fiscal year 2009, Repligen has paid $1.1 million to the University of Michigan.
CTLA4-Ig’s mechanism of action is different from the current therapies for autoimmune disease or organ transplant rejection, thus it may provide a treatment for patients who are refractory to existing therapies.

 In December 2005, the FDA approved Bristol’s application to market CTLA4-Ig, under the brand
name Orencia®, for treatment of rheumatoid arthritis. In January 2006, Repligen and the University of Michigan jointly filed a lawsuit against Bristol in the United States District Court
for the Eastern District of Texas for patent infringement. In April 2008, Repligen and the University of Michigan entered into a settlement agreement with Bristol pursuant to which, Bristol made an initial payment of $5 million to Repligen and will
pay us royalties on the U.S. net sales of Orencia® for any clinical indication at a rate of 1.8% for the first $500 million of annual sales, 2.0% for the next $500 million and 4% of annual
sales in excess of $1 billion for each year from January 1, 2008 until December 31, 2013.”

 3

 Definitive Proxy Statement

 Annual Cash Incentive Compensation, page 19

8.
We note your response to our prior comment 8. Please supplementally provide us with a detailed analysis of why you believe public disclosure of the requested information, based
upon your specific facts and circumstances, would cause the company competitive harm. Your analysis in your response letter should clearly describe each target, describe how it is used in the calculation of performance, and explain how such
information is likely to cause competitive harm, as opposed to conclusory statements that the information would represent a competitive disadvantage. Unless you disclose this information to the staff it is not possible for us to determine whether
omission of the information from your 10-K disclosure is appropriate. Please note that you may request Rule 83 confidential treatment for portions of your response letter that contain commercially sensitive information.

In the alternative, please provide complete disclosure of the specific targets against which company and individual performance was measured and the
calculations used to determine and subsequently measure performance. The disclosure should allow the reader to understand how the Compensation Committee determined the percentage achievement of individual and corporate goals.

 RESPONSE: We respectfully advise the Staff that we propose to modify the presentation regarding annual cash incentive in our definitive proxy statement
for the year ended March 31, 2009. For reference, we would have updated our disclosure for the year ended March 31, 2008 beginning at the bottom of page 21, which has been presented below. In addition to the newly presented table below,
all proposed changes or additions to such disclosure to incorporate the Staff’s recommendations are highlighted in bold italic print:

 “Achievement of objectives and cash incentive determination:

 The portion of
cash incentive tied to corporate results is determined based on evaluation of the percentage completion of the established corporate objectives determined at the beginning of the fiscal year. The ultimate evaluation of achievement is at the
sole discretion of the Compensation Committee following discussions with management and is based both on quantitative, objective calculations such as the achievement of certain financial or research milestones, as well as a qualitative assessment
which takes into consideration the level of effort, end results, and other contributing internal and external factors that could reasonably be expected to evaluate performance. The objectives are designed to be difficult to achieve 100%
completion. These objectives include financial performance objectives (30% of total), product development objectives (30% of total), intellectual property objectives (20% of total) and business and organizational development objectives (20% of
total). The Chief Executive Officer determined and the Compensation Committee agreed that the aggregate percentage completion of the objectives for financial performance, product development, intellectual property and business and organizational
development was 86.3%. The corporate portion of cash incentive for all officers was paid calculated at this level of achievement.

 4

 The following is a summary of the fiscal year 2008 Corporate Objectives and the
detailed calculation supporting the achievement of each objective:

 2008 Company Goals

Weight

Percent Achieved
(as determined by
Compensation
Committee)

Weight
achieved

 Financial Performance

 Achieve certain financial performance targets, including total sales in excess of $17.3 million, gross margin in excess of 68%, net loss less
than $3.7 million and end of year cash and investments of greater than $19.0 million

30.0
%

95.0
%

28.5
%

 Product Development

 Bioprocessing product and business development goals included executing certain supply agreements and product development objectives and
supporting key customer requirements

6.0
%

82.0
%

4.9
%

 RG1068 for pancreatic imaging goals included initiating a phase 3 clinical trial, negotiating the NDA requirements with FDA, obtaining FDA
fast track designation, and beginning the process to qualify a marketing partner.

6.0
%

81.0
%

4.9
%

 RG2417 for bipolar depression goals included completing a phase 2a study, designing a phase 2b study, and completing additional assessments of
uridine in other disease models

6.0
%

83.0
%

5.0
%

 Preclinical research objectives included assessing the suitability of our existing clinical candidate for HDAC inhibitors for
Friedreich’s Ataxia, as well as researching the activity of this candidate in additional animal models of disease, and seeking financial backing to support ongoing research activities. Further objectives included assessing both opportunities to
outlicense existing candidates and identify and potentially in-license new product candidates.

12.0
%

73.5
%

8.8
%

 Maximize the value of intellectual property portfolio

 Complete the Imclone and Bristol-Myers Squibb litigations resulting in the best possible outcome for Repligen.

20.0
%

92.5
%

18.5
%

 Business and Organizational Development

 Update strategic plan; successfully recruit new members of management team and board of directors; maintain effective internal controls and
ensure compliance with SEC requirements; other miscellaneous objectives.

20.0
%

78.5
%

15.7
%

 TOTAL CORPORATE PERFORMANCE

100.0
%

86.3
%

 5

 Individual Objectives

 In addition to the corporate objectives outlined above, executive officers with the exception of Dr. Herlihy also have an individual objective component to their annual cash incentive compensation. The performance assessment for
the individual objectives of the executive officers is not calculated on a line item basis, but rather represents an overall assessment as to how the officer contributed to the success of the company within their area of responsibility. The
individual objectives are designed to be difficult to achieve 100% completion.

 Dr. Rusche is ultimately responsible for providing
leadership to all of Repligen’s research and development activities. Specifically, his objectives included leading our research efforts surrounding HDAC inhibitors for the treatment of Friedreich’s ataxia, including assessing the
suitability of our existing clinical candidate and evaluating the effectiveness of this candidate in other disease models; driving process development work to assist the growth of our bioprocessing business; supporting our business development
initiatives; developing and implementing our strategic and financial plan; representing Repligen to the investment and scientific community as necessary; and otherwise assisting in personnel recruitment, development and morale. The overall
evaluation of achievement in these factors is both quantitative and qualitative. The Chief Executive Officer determined and the compensation committee agreed that the aggregate achievement of the individual objectives for Dr. Rusche was 85.0%.

 Dr. Witt is ultimately responsible for Repligen’s intellectual property portfolio as well as all manufacturing operations.
Specifically, his objectives included maximizing the value of our intellectual property portfolio through the Imclone and Bristol-Myers Squibb litigations; delivering on our gross margin targets, ensuring product quality and on-time delivery to
customers; supporting our business development initiatives; developing and implementing our strategic and financial plan; representing Repligen to the investment community as necessary; and oth

Via Facsimile and U.S. Mail
Mail Stop 6010
                                                                                                 April 16, 2009   Walter C. Herlihy, Ph.D. President, Chief Executive Officer and Director 41 Seyon Street, Building #1, Suite 100 Waltham, Massachusetts 02453
Re: Repligen Corporation
  Supplemental Response to Form 10-K for the Year
Ended March 31, 2008
  Filed March 31, 2009   File No. 000-14656

Dear Dr. Herlihy:

We have reviewed your filing and have the following comments.  In our
comments, we ask you to provide us with  information to better understand your
disclosure.  Where it requests you to revi se disclosure, the information you provide
should show us what the revised disclosure  will look like and identify the annual or
quarterly filing, as applicable, in which you intend to first include it.  If you do not
believe that revised disclosure  is necessary, explain the reason in your response.  After
reviewing the information provided, we may raise additional comments and/or request
that you amend your filing.   Please understand that the purpose of our re view process is to assist you in your
compliance with the applicable disclosure  requirements and to  enhance the overall
disclosure in your filing.  We look forward to  working with you in these respects.  We
welcome any questions you may have about our comments or on any other aspect of our review.  Feel free to call us at the telephone numbers listed at the end of this letter.

Form 10-K for year ended March 31, 2008

Item 1. Business, page 3
1. a. In your response to our prior comment 1 you state that Applied’s purchases of
Protein A from the Company account for “less 10% of the Company’s product
revenues and less than 4% of total revenues.”  However, we note that your
disclosure on page 5 of the 10-K under “Significant Customers” states that
Applied was one of two customers whose purchases of Protein A products

Walter C. Herlihy, Ph.D.
Repligen Corporation
April 16, 2009 Page 2
accounted for more than 10% of the Company’s total revenues in 2008, 2007
and 2006.  Please explain this apparent discrepancy.
b. In addition, you state that the supply ag reement does not obligate the parties to
sell or purchase specific qua ntities of Protein A.  Yet, you also state that
Repligen and Applied are in fact obligat ed with respect to the quantities of
Protein A reflected in Applied’s short-term forecasts of its Protein A needs and these amounts are not fixed.  So, while the amounts of Protein A under
the supply agreement may have been imma terial so far, it seems possible that
they could become much larger in the future.  If true, while the agreement might qualify as immaterial right no w, depending on the revenues involved
thus far, it could become material in th e future.  Please inform us whether this
is the case and, if so, that you will file the agreement as a material contract
should revenues attributable to the contract become material.
2. We note your response to our prior comment  3.  Please also provide the following
material information regarding the ex clusive license agreement between the
company and the University of Michigan:
a. Total amounts received to date under the agreement;
b. Any annual fees, if applicable;
c. The number of additional products you anticipate you will develop under
this agreement; and
d. The term and termination provisions, including any payments the company
would be required to make in  the event of termination.

Definitive Proxy Statement

Annual Cash Incentive Compensation, page 19
3. We note your response to our prior comment  8.  Please supplementally provide us
with a detailed analysis of why you believe public disc losure of the requested
information, based upon your specific facts and circumstances, would cause the
company competitive harm.  Your analysis  in your response letter should clearly
describe each target, describe  how it is used in the cal culation of performance,
and explain how such information is likel y to cause competitive harm, as opposed
to conclusory statements that the in formation would represent a competitive
disadvantage.  Unless you disclose this in formation to the staff it is not possible
for us to determine whether omission of the information from your 10-K disclosure is appropriate.  Please note that you may re quest Rule 83 confidential

Walter C. Herlihy, Ph.D.
Repligen Corporation
April 16, 2009 Page 3
treatment for portions of your response le tter that contain co mmercially sensitive
information.
In the alternative, please provide complete disclosure  of the specific targets
against which company and individual  performance was measured and the
calculations used to determine and s ubsequently measure performance. The
disclosure should allow the reader to understand how the Compensation
Committee determined the percentage of achievement of individual and corporate
goals.

*    *    *    *

Please provide us the information request ed within 10 busine ss days or tell us
when you will provide us with a response.  Please furnish a cover letter with your response that keys your responses to our  comments and provide any requested
information.  Detailed cover letters greatly facilitate our  review.  Please furnish your
letter on EDGAR under the form type label CORRESP.

 We urge all persons who are responsible for the accuracy and adequacy of the
disclosure in the filing to be certain that the filing includes all in formation required under
the Securities Exchange Act of 1934 and th at they have provided all information
investors require for an informed invest ment decision.  Since the company and its
management are in possession of all facts re lating to a company’s disclosure, they are
responsible for the accuracy and adequacy of the disclosures they have made.
  In connection with responding to our co mments, please provide, in your letter, a
statement from the company acknowledging that:
• the company is responsible for the adequacy  and accuracy of the disclosure in the
filing;
• staff comments or changes to disclosure  in response to staff comments do not
foreclose the Commission from taking any action with respect to the filing; and
• the company may not assert staff comme nts as a defense in any proceeding
initiated by the Commission or any person under the federal secu rities laws of the
United States.

In addition, please be advise d that the Division of Enfo rcement has access to all
information you provide to the staff of the Divi sion of Corporation Fi nance in our review
of your filing or in response to our comment on your filing.

Walter C. Herlihy, Ph.D.
Repligen Corporation April 16, 2009 Page 4
 Please contact Laura Crotty, Staff Atto rney, at (202) 551-3563, Dan Greenspan,
Special Counsel, at (202) 551-36231 or myself at (202) 551-3715 if you have questions regarding the processing of your response as well as any questions regarding comments
and related matters.

Sincerely,

Jeff Riedler Assistant Director

CORRESP
1
filename1.htm

Correspondence

 Repligen Corporation

 41 Seyon Street, Building #1, Suite 100

 Waltham, Massachusetts 02453

 March 31, 2009

 United States Securities and Exchange
Commission

 Division of Corporation Finance

 100 F Street N.E.

 Washington, D.C. 20549

 Attention:  Jeffrey
Riedler, Assistant Director

RE:

Repligen Corporation

Form 10-K for the Year Ended March 31, 2008

Filed June 13, 2008

File No. 000-14656

 Dear Mr. Riedler:

 This letter is being furnished in response to the comments of the Staff (the “Staff”) of the Securities and Exchange Commission (the “Commission”) in the Commission’s letter dated
March 2, 2009 (the “Comment Letter”) to Dr. Walter C. Herlihy, Ph.D., President, Chief Executive Officer and Director of Repligen Corporation (“Repligen” or the “Company”), with respect to the
Company’s Annual Report on Form 10-K for the fiscal year ended March 31, 2008. The response set forth below has been organized in the same manner in which the Commission’s comments and headings were organized in the Comment Letter.

 Form l0-K for year ended March 31, 2008

 Item 1. Business, page 3

1.
Please file as an exhibit to the Form l0-K the supply agreement between the company and Applied Biosystems, Inc.

 RESPONSE: We respectfully advise the Staff that the Company has not filed the supply agreement with Applied Biosystems, Inc. (“Applied”) because
the Company does not believe that it is a material contract pursuant to Item 601(b)(10) of Regulation S-K. The supply agreement is such as ordinarily accompanies the business of the Company and does not fall into any of the subcategories
contained in Item 601(b)(10)(ii). The supply agreement relates to the sale of Protein A, one of the Company’s products. The supply agreement itself does not obligate Applied to purchase any specific quantity of Protein A from the Company
nor does it obligate the Company to sell any specific quantity of Protein A to Applied. Pursuant to the supply agreement, Applied periodically provides

the Company with a medium term forecast (less than six months) of its need for Protein A on an informational basis, again without a requirement of either
party to buy or sell Protein A. Applied also provides short term forecasts of its Protein A needs, covering a period less than three months. For the first three years of the supply agreement, Applied must buy and the Company must sell Applied 100%
of such short term forecast. For the fourth year of the supply agreement, Applied must buy and the Company must sell to Applied 50% of such short term forecast. The price of such purchases is determined by a matrix set forth in the supply agreement
and is based on volume. While the amount varies from year to year, Applied’s purchases of Protein A from the Company constitute less than 10% of the Company’s product revenues and less than 4% of total revenues. Therefore, the Company does
not believe that the supply agreement with Applied is a material contract required to be filed as an exhibit to its Form 10-K.

2.
We note your discussion in this section of the exclusive license agreement between the company and the Massachusetts Institute of Technology on page 4. Please describe the
material terms of this agreement in this section and file the agreement as an exhibit to the Form 10-K pursuant to Item 60l(b)(10) of Regulation S-K. Alternatively, please provide us with an analysis supporting your determination that the
agreement is not material to the company.

 RESPONSE: We respectfully advise the Staff that the license agreement with the
Massachusetts Institute of Technology (“MIT”) dated May 4, 2004 terminated on the last to expire patents covered by such license. The last to expire patent expired in May 2005. The patents covered by the license were related to the
use of DNA enhancers. Pursuant to the license, the Company was required to pay a certain percentage of any amounts received by the Company from third parties for use of the intellectual property covered by the license. For at least the last 10
years, the technology covered by the license was not relevant to the Company’s core operations. Since May 2005, the license merely acted as a requirement to pay money, if any, upon a successful outcome in the litigation described below.

 Prior to the termination of the License, in May 2004, the Company and MIT sued ImClone Systems, Incorporated (“ImClone”) claiming
infringement of the patents which were the subject of the license. In settlement of such claims, ImClone agreed to pay the Company and MIT $65 million of which approximately $11 million, or 17%, was paid to MIT pursuant to the terms of the license.
The Company has no further obligations or other responsibilities to MIT under this license.

 The Company is currently engaged in the
development and sale, if approved for sale, of (i) Protein A which is used in the manufacture of monoclonal antibodies, (ii) synthetic hormones which, among other things, may assist in the diagnosis of pancreatitis and related disorders,
and (iii) therapeutic product candidates for, among other things, the treatment of bipolar disorder and Friedreich’s ataxia. The Company was incorporated in 1981 and has always been focused, among other things, on the research and
development of therapeutic candidates for various diseases. The Company has frequently in-licensed various technologies and inventions, including those covered by patents. The in-license

 2

of patents is one that ordinarily accompanies the business of researching and developing therapeutic product candidates as contemplated by
Item 601(b)(10) of Regulation S-K and does not fall into any of the subcategories contained in Item 601(b)(10)(ii). In addition, all patent rights under the license have expired and no further payments are due under the license. Therefore,
the Company does not believe the license from MIT is a material contract required to be filed as an exhibit to its Form 10-K.

3.
We note your discussion of the exclusive license agreement between the company and the University of Michigan on page 4, which agreement has been filed as a material contract and
incorporated by reference to your 10-K. Please describe the material terms of the agreement in this section of the 10-K.

 RESPONSE: We respectfully advise the Staff that we propose to include in the Company’s annual report on Form 10-K for the year ended March 31, 2009, the following disclosure regarding the material terms of the license agreement
with the University of Michigan. All proposed changes to such disclosure compared to previously submitted disclosures in our annual report on Form 10-K for the year ended March 31, 2008 to incorporate the Staff’s recommendations are
highlighted in bold italic print:

 “CTLA4-Ig

 CTLA4 is a key regulator of the activity of the immune system. CTLA4 “turns off” the immune system after it has successfully cleared a bacterial
or viral infection by blocking the activation of T-cells, the immune cells responsible for initiating an immune response. In the 1990’s, our collaborators at the University of Michigan and the U.S. Navy demonstrated in animal models that a
fusion protein consisting of fragments of CTLA4 and an antibody (“CTLA4-Ig”) could be used to treat certain autoimmune diseases. This research finding resulted in the granting of U.S. patent No. 6,685,941 (“the ‘941
Patent”) covering the treatment of certain auto-immune disorders including rheumatoid arthritis with CTLA4-Ig. The ‘941 Patent is owned by the University of Michigan and exclusively licensed to Repligen. In consideration of this
exclusive license, Repligen agreed to pay the University of Michigan 15% of all royalty income received, after deducting legal expenses. CTLA4-Ig’s mechanism of action is different from the current therapies for autoimmune disease or
organ transplant rejection, thus it may provide a treatment for patients who are refractory to existing therapies.

 In December 2005, the FDA approved Bristol’s application to market CTLA4-Ig, under the brand name Orencia®, for treatment of rheumatoid arthritis. In
January 2006, Repligen and the University of Michigan jointly filed a lawsuit against Bristol in the United States District Court for the Eastern District of Texas for patent infringement. In April 2008, Repligen and the University of Michigan
entered into a settlement agreement with Bristol pursuant to which, Bristol made an initial payment of $5 million to Repligen and will pay us royalties on the U.S. net sales of Orencia® for
any clinical indication at a rate of 1.8% for the first $500 million of annual sales, 2.0% for the next $500 million and 4% of annual sales in excess of $1 billion for each year from January 1, 2008 until December 31, 2013.”

 3

4.
We note your discussion of the settlement agreement between the company and Bristol-Myers Squibb on pages 4, 6 and 55. Please file this agreement as an exhibit to the Form l0-K
pursuant to Item 60l(b)(10) of Regulation S-K.

 RESPONSE: We respectfully advise the Staff that the Settlement and
Release Agreement between the Company, the Regents of the University of Michigan and Bristol-Myers Squibb Company was executed and became effective on April 7, 2008 (the “Bristol Agreement”), after the end of our fiscal year 2008
which ended on March 31, 2008. Item 601(a)(4) of Regulation S-K requires the Company to file as exhibits to Form 10-K material contracts, pursuant to Item 601(b)(10), a material contract which is executed or becomes effective during
the reporting period reflected by the Form 10-Q or Form 10-K. Since the Bristol Agreement was executed after the reporting period reflected in the Form 10-K it was not filed as an exhibit thereto. We advise the Staff that the execution of the
Bristol Agreement was disclosed pursuant to Item 1.01 of a Form 8-K which was filed on April 11, 2008 and the Bristol Agreement itself was filed as Exhibit 10.1 to the Company’s Form 10-Q for the three month period ending
June 30, 2008 which was filed on August 8, 2008. We further advise the Staff that we intend to incorporate by reference the Bristol Agreement as an exhibit to the Company’s Form 10-K for the year ending March 31, 2009 as required
by Item 601 of Regulation S-K.

 Item 5. Market for Registrant’s Common Equity, Related Stockholder Matters and Issuer Purchases of
Equity Securities, page 14

5.
Please revise to disclose the information required by Item 201(e) of Regulation S-K.

 RESPONSE: We respectfully advise the Staff that pursuant to Instruction 7 to Item 201(e), the Company has included the performance graph required by
Item 201(e) of Regulation S-K in an annual report to security holders required by Exchange Act Rule 14a-3 (the “2008 Annual Report”) that accompanied the registrant’s proxy relating to the Company’s 2008 annual meeting of
security holders at which directors were to be elected. Pursuant to Rule 14a-3(b)(9), the performance graph required by Item 201(e) of Regulation S-K is required to be included in an annual report to shareholders. In lieu of providing
shareholders with the Company’s Annual Report on Form 10-K in satisfaction of the requirements of Rule 14a-3, the Company delivered the 2008 Annual Report which met the conditions required therein. As required by Rule 14a-3(c), seven copies of
the Company’s 2008 Annual Report were filed with the Commission. For our 2009 annual report to shareholders, we intend to include the performance graph as we did in the 2008 Annual Report.

 Notes to Consolidated Financial Statements

 11. Scripps
Agreement, page 54

6.
 You state that, pursuant to the company’s license agreement with The Scripps Research Institute, Repligen has agreed to make certain additional payments in
the event it achieves specified developmental and commercial milestones. Notwithstanding that the

 4

company has been granted confidential treatment for portions of this agreement, including the amount of specific potential milestone payments, please
disclose the aggregate potential milestone payments to be made under the agreement, Alternatively, please provide your analysis why these aggregate potential payments are not material.

 RESPONSE: We respectfully agree with the Staff’s recommendation that further disclosure of the aggregate potential milestone payments is appropriate.
In response to the Staff’s comment, we would have made the following revised disclosure as of March 31, 2008 and are planning to include equivalent disclosure in our future filings on form 10-K. We have included the first two paragraphs of
the related disclosure as of March 31, 2008 below for reference. All proposed changes or additions to such disclosure to incorporate the Staff’s recommendations are highlighted in bold italic print:

 “11. Scripps Agreements

 License Agreement

 On April 6, 2007 (“the Effective Date”), the Company entered into an
exclusive worldwide commercial license agreement (“License Agreement”) with The Scripps Research Institute (“Scripps”). Pursuant to the License Agreement, the Company obtained a license to use, commercialize and sublicense
certain patented technology and improvements thereon, owned or licensed by Scripps, relating to compounds which may have utility in treating Friedreich’s Ataxia, an inherited neurodegenerative disease. Research in tissues derived from patients,
as well as in mice, indicates that the licensed compounds increase production of the protein frataxin, which suggests potential utility of these compounds in slowing or stopping progression of the disease. There are currently no approved treatments
for Friedreich’s ataxia.

 Pursuant to the License Agreement, the Company agreed to pay Scripps an initial license fee
of $300,000, certain royalty and sublicense fees and, in the event the Company achieves specified developmental and commercial milestones, certain additional milestone payments. Total future milestone payments, were all milestones to be
achieved, would be approximately $4.3 million. In addition, the Company issued Scripps and certain of its designees 87,464 shares of the Company’s common stock (the “Shares”) representing $300,000 as of the Effective Date. The
Company recorded the initial license payment and the value of the shares issued as research and development costs in the Company’s statement of operations in fiscal 2008.”

 Definitive Proxy Statement

 Compensation Discussion and Analysis, page 18

7.
 We note your statements on page 18 that the Compensation Committee set total executive officer compensation in fiscal 2008 based on the Radford Biotechnology
Survey. Please revise your discussion to identify the companies from the survey that were reviewed by

 5

the Compensation Committee and used as a basis for the salaries paid to Repligen’s executive officers. If the companies are too numerous to list,
please disclose the criteria for including these companies in the Survey.

 RESPONSE: We respectfully agree with the
Staff’s recommendation that further identification of the criteria used for inclusion in the Radford Biotechnology Survey is appropriate. In response to the Staff’s comment, we would have revised our Definitive Proxy Statement and are
planning to include equivalent disclosure in future filings as appropriate. We have included the first two paragraphs of the related disclosure as of March 31, 2008 below for reference. All proposed changes or additions to such disclosure to
incorporate the Staff’s recommendations are highlighted in bold italic print:

 “Executive Compensation

 Base Salary. Each executive officer (except the Chief Executive Officer whose performance is reviewed by the
Compensation Committee) has an annual performance review with the Chief Executive Officer, who makes recommendations on salary increases, promotions and equity grants to the Compensation Committee. The recommended salary increases are based on the
executive officer’s qualifications, performance, experi

CORRESP
1
filename1.htm

Correspondence

Repligen Corporation

41 Seyon Street, Bldg #1

Waltham, MA 02453-8335

Telephone: 781-250-0111

Telefax:     781-250-0115

 March 13, 2009

 United States
Securities and Exchange Commission

 Division of Corporation Finance

 100 F Street, N.E.

 Washington, D.C. 20549-7010

 Attention: Jeff Riedler / Laura Crotty

Re:

Repligen Corporation

Form 10-K for Fiscal Year Ended March 31, 2008 : File No. 000-14656

 Dear Mr. Riedler and Ms. Crotty:

 This letter is being furnished on behalf of Repligen Corporation (the “Company”) in response to comments in the letter dated March 2, 2009 (the “Letter”) from Jeff Riedler of the
Staff (the “Staff”) of the Securities and Exchange Commission (the “Commission”) to Walter C. Herlihy, President and Chief Executive Officer of the Company, with respect to the Company’s Annual Report on Form
10-K for the fiscal year ended March 31, 2008 (the “Form 10-K”) that was filed with the Commission on June 13, 2008.

 We
respectfully request an extension of time until March 31, 2009 to provide our response. This will provide the time needed to fully and adequately prepare and review the response with all applicable parties. Thank you for your consideration in
this matter.

 If you require additional information, please telephone the undersigned at (781) 419-1874.

Sincerely,

 /s/ William J. Kelly

William J. Kelly

Chief Financial Officer

Repligen Corporation

cc:

Walter C. Herlihy, Repligen Corporation

Robert E. Puopolo, Goodwin Procter LLP

Via Facsimile and U.S. Mail

Mail Stop 6010
                                                                                                 March 2, 2009   Walter C. Herlihy, Ph.D. President, Chief Executive Officer and Director 41 Seyon Street, Building #1, Suite 100 Waltham, Massachusetts 02453
Re: Repligen Corporation
  Form 10-K for the Year Ended March 31, 2008   Filed June 13, 2008   File No. 000-14656

Dear Dr. Herlihy:

We have reviewed your filing and have the following comments.  In our
comments, we ask you to provide us with  information to better understand your
disclosure.  Where it requests you to revi se disclosure, the information you provide
should show us what the revised disclosure  will look like and identify the annual or
quarterly filing, as applicable, in which you intend to first include it.  If you do not
believe that revised disclosure  is necessary, explain the reason in your response.  After
reviewing the information provided, we may raise additional comments and/or request
that you amend your filing.   Please understand that the purpose of our re view process is to assist you in your
compliance with the applicable disclosure  requirements and to  enhance the overall
disclosure in your filing.  We look forward to  working with you in these respects.  We
welcome any questions you may have about our comments or on any other aspect of our review.  Feel free to call us at the telephone numbers listed at the end of this letter.

Form 10-K for year ended March 31, 2008

Item 1. Business, page 3
1. Please file as an exhibit to the Form  10-K the supply agreement between the
company and Applied Biosystems, Inc.
2. We note your discussion in this sectio n of the exclusive license agreement
between the company and the Massachuset ts Institute of Technology on page 4.
Please describe the material terms of this agreement in this section and file the

Walter C. Herlihy, Ph.D.
Repligen Corporation
March 2, 2009 Page 2
agreement as an exhibit to the Form 10-K pursuant to Item 601(b)(10) of
Regulation S-K.  Alternatively, please pr ovide us with an analysis supporting
your determination that the agreement is not material to the company.
3. We note your discussion of the exclusive license agreement between the company
and the University of Michigan on page 4, which agreement has been filed as a material contract and incorporated by re ference to your 10-K.  Please describe the
material terms of the agreement in this section of the 10-K.
4. We note your discussion of the settleme nt agreement between the company and
Bristol-Myers Squibb on pages 4, 6 and 55.  Please file this agreement as an
exhibit to the Form 10-K pursuant to Item 601(b)(10) of Regulation S-K.
 Item 5. Market for Registrant’s Common Equ ity, Related Stockholder Matters and Issuer
Purchases of Equity Securities, page 14
5. Please revise to disclose the informati on required by Item 201(e) of Regulation S-
K.

Notes to Consolidated Financial Statements

11. Scripps Agreement, page 54
6. You state that, pursuant to the company’ s license agreement with The Scripps
Research Institute, Repligen has agreed to make certain additional payments in
the event it achieves specified developmental and commercial milestones.
Notwithstanding that the company has been  granted confidential treatment for
portions of this agreement, including th e amount of specific potential milestone
payments, please disclose the aggregate pot ential milestone payments to be made
under the agreement.  Alternatively, please provide your analysis why these aggregate potential payments are not material.

Definitive Proxy Statement

Compensation Discussion and Analysis, page 18
7. We note your statements on page 18 that the Compensation Committee set total
executive officer compensation in fiscal 2008 based on the Radford Biotechnology Survey.  Please revise your discussion to identify the companies from the survey that were reviewed by the Compensation Committee and used as

Walter C. Herlihy, Ph.D.
Repligen Corporation
March 2, 2009 Page 3
a basis for the salaries paid to Repligen’s  executive officers. If the companies are
too numerous to list, please disclose the criteria for including these companies in
the Survey.
 Annual Cash Incentive Compensation, page 19

8. We note your discussion concerning the pe rformance criteria c onsidered by the
Compensation Committee in making its awards for fiscal year 2008, which was comprised of company and individual objectives.
(a) With respect to company performance objectives, your disclosure is
too general.  Please disclose th e specific targets against which
company performance was measured.  Please note that disclosure of
these specific targets is warranted unless the company believes that
competitive harm would result from such disclosure, in which case it
may omit disclosure of the specific ta rgets as long as it complies with
Instruction 4 to 402(b).  In accordance with that instruction, please supplementally explain to us your basis for keeping the individual performance target information c onfidential.  Please explain why,
based upon your specific facts and ci rcumstances, publicly releasing
this information will cause Impax competitive harm; and
(b) With respect to individual executive performance, to the extent that the
performance criteria you have listed  was subject to quantification for
purposes of the Compensation Committee’s performance evaluation, please provide the metrics used.  If not, please explain how the
Compensation Committee determined th e percentage of completion of
individual results, as described on pages 21 and 22.

Equity Compensation Plan Information, page 35

9. Please revise the equity compensation plan  table to reflect information as of
March 31, 2008, the end of the most recently completed fiscal year, rather than as
of December 31, 2007, pursuant to Item 201(d) of Regulation S-K.

*    *    *    *

Please provide us the information request ed within 10 busine ss days or tell us
when you will provide us with a response.  Please furnish a cover letter with your response that keys your responses to our  comments and provide any requested
information.  Detailed cover letters greatly facilitate our  review.  Please furnish your
letter on EDGAR under the form type label CORRESP.

Walter C. Herlihy, Ph.D.
Repligen Corporation March 2, 2009 Page 4
  We urge all persons who are responsible for the accuracy and adequacy of the
disclosure in the filing to be certain that the filing includes all in formation required under
the Securities Exchange Act of 1934 and th at they have provided all information
investors require for an informed invest ment decision.  Since the company and its
management are in possession of all facts re lating to a company’s disclosure, they are
responsible for the accuracy and adequacy of the disclosures they have made.
  In connection with responding to our co mments, please provide, in your letter, a
statement from the company acknowledging that:
• the company is responsible for the adequacy  and accuracy of the disclosure in the
filing;
• staff comments or changes to disclosure  in response to staff comments do not
foreclose the Commission from taking any action with respect to the filing; and
• the company may not assert staff comme nts as a defense in any proceeding
initiated by the Commission or any person under the federal secu rities laws of the
United States.

In addition, please be advise d that the Division of Enfo rcement has access to all
information you provide to the staff of the Divi sion of Corporation Fi nance in our review
of your filing or in response to our comment on your filing.
Please contact Laura Crotty, Staff Attorne y, at (202) 551-3563 or myself at (202)
551-3715 if you have questions regarding the pr ocessing of your res ponse as well as any
questions regarding comments and related matters.

Sincerely,

Jeff Riedler Assistant Director

Via Facsimile and U.S. Mail
Mail Stop 6010
        May 23, 2007

Mr. Walter C. Herlihy
Chief Executive Officer and President
Repligen Corporation
41 Seyon Street
Building 1, Suite 100
Waltham, MA   02453

Re: Repligen Corporation
 Form 10-K for Fiscal Year Ended March 31, 2006
File No. 0-14656

Dear Mr. Herlihy:

 We have completed our review of your Form 10-K and have no further comments
at this time.

        S i n c e r e l y ,

James Atkinson
Branch Chief

CORRESP
1
filename1.htm

Response Letter

 April 5, 2007

 United States Securities and Exchange Commission

 Division of Corporation Finance

 100 F Street, N.E.

 Washington, D.C. 20549-7010

 Attention: Jim B. Rosenberg

Re:
Repligen Corporation

 Form 10-K for Fiscal Year
Ended March 31, 2006

 File No. 0-14656

 Dear Ladies and Gentlemen:

 This letter is being furnished on behalf of Repligen Corporation (the “Company”) in response to
comments in the letter dated March 5, 2007 (the “Letter”) from Jim B. Rosenberg of the Staff (the “Staff”) of the Securities and Exchange Commission (the “Commission”) to Walter C. Herlihy,
President and Chief Executive Officer of the Company, with respect to the Company’s Annual Report on Form 10-K for the fiscal year ended March 31, 2006 (the “Form 10-K”) that was filed with the Commission on June 9,
2006.

 The responses and supplementary information set forth below have been organized in the same manner in which the Commission’s comments were
organized. The text of the Staff’s comments are copied below in italics and indented for your reference. Copies of this letter are being sent under separate cover to Mark Brunhofer of the Commission.

 Form 10-K for the fiscal year ended March 31, 2006

 Management’s Discussion and Analysis of Financial Condition and Results of Operations

 Critical Accounting
Estimates, page 20

1.
Your revenue recognition disclosure does not appear to disclose the judgments you must make each period and the uncertainties surrounding those judgments. In addition, your
revenue recognition, inventory reserve and accrued liabilities disclosures do not appear to discuss the potential variability of reasonably likely changes in your underlying estimates nor do your disclosures appear to discuss how accurate your
estimates have been in prior periods. Please provide us in a disclosure-type format, revised discussions of your critical accounting estimates that discuss specifically the judgments you make, why your estimates or assumptions bear the risk of
change, the impact on your financial results, financial condition and liquidity of reasonably likely changes in the underlying assumptions and the extent to which actual subsequent experience has differed materially from your initial estimates in
each of the periods presented. Please see FR-72.

 Jim B. Rosenberg

 April 5,
2007

  Page
 2

 RESPONSE:

 We agree with the Staff’s recommendation that further discussion of our critical accounting policies is appropriate. In response to the Staff’s comment, we
would have made the following revised disclosure as of March 31, 2006 and we are planning to include equivalent disclosure in our future filings on Form 10-K. We have included the entirety of the related disclosure as of March 31, 2006
below for completeness. All proposed changes or additions to such disclosure to incorporate the Staff’s recommendations are highlighted in bold italic print.

 Revenue Recognition

 We apply Staff Accounting Bulletin No. 104, “Revenue Recognition” (“SAB
No. 104”) to our revenue arrangements.

 We generate product revenues from
the sale of our Protein A products to customers in the pharmaceutical and process chromatography industries and from the sale of SecreFlo® to hospital-based gastroenterologists. In accordance with SAB No. 104, we recognize revenue related to product sales upon delivery of the product to the customer as long as there is persuasive
evidence of an arrangement, the sales price is fixed or determinable and collection of the related receivable is reasonably assured. Determination of whether these criteria have been met are based on management’s judgments primarily
regarding the fixed nature of the fee charged for product delivered, and the collectibility of those fees. We have a few longstanding customers who comprise the majority of our revenue and have excellent payment history. We have had no significant
write-offs of uncollectible invoices in the periods presented. Should changes in conditions cause management to determine that these criteria are not met for certain future transactions, revenue recognized for any reporting period could be adversely
affected.

 At the time of sale, we also evaluate the need to accrue for warranty and sales returns. The supply agreements we have with our
customers and related purchase orders identify the terms and conditions of each sale and the price of the goods ordered. Due to the nature of our sales arrangements, inventory produced for sale is tested for quality specifications prior to shipment.
Since the product is manufactured to order and in compliance with required specifications prior to shipment, the likelihood of sales return, warranty or other issues is largely diminished. Sales returns and warranty issues are infrequent and have
had nominal impact on our financial statements historically. Should changes in conditions cause management to determine that warranty, returns or other sale-related reserves are necessary for certain future transactions, revenue recognized for any
reporting period could be adversely affected.

 Jim B. Rosenberg

 April 5,
2007

  Page
 3

 During the fiscal
year ended March 31, 2006, we received $310,000 of cash from a sponsored research and development project under an agreement with the Stanley Medical Research Institute (SMRI). Research revenue is recognized when the expense has been
incurred and services have been performed. Determination of which costs incurred qualify for reimbursement under the terms of our contractual agreement and the timing of when such costs were incurred involves the judgment of management. We believe
our calculations are based upon the agreed-upon terms as stated in our arrangement. However, should our estimated calculations change or be challenged by SMRI, research revenue may be adjusted in subsequent periods. Our calculations have not
historically changed or been challenged and we do not anticipate any subsequent change in our revenue related to this sponsored research and development project.

 During fiscal 2004 we generated non-product revenues from sponsored research and development projects under a Small Business Innovation Research (“SBIR”) Phase I grant. Research expenses in the accompanying
statements of operations include funded and unfunded expenses. Additionally, during fiscal year 2006, the Company earned and recognized approximately $72,000 in royalty revenue from ChiRhoClin, Inc. Revenues earned from ChiRhoClin royalties
are recorded in the periods when they are earned based on royalty reports sent by ChiRhoClin to the Company.

 There have been no material
changes to our initial estimates related to revenue recognition in any periods presented in the accompanying financial statements.

 Inventory

 Inventories relate to our Protein A business. We value inventory at cost or, if lower, fair market value. We determine cost using the
first-in, first-out method. We review our inventories at least quarterly and record a provision for excess and obsolete inventory based on our estimates of expected sales volume, production capacity and expiration dates of raw
materials, work-in process and finished goods. Expected sales volumes are determined based on supply forecasts provided by our key customers for the next three to twelve months. We write down inventory that has become obsolete, inventory
that has a cost basis in excess of its expected net realizable value, and inventory in excess of expected requirements to cost of goods sold. Manufacturing of Protein A finished goods is done to order and tested for quality specifications
prior to shipment.

 A change in the estimated timing or amount of demand for our products could result in additional provisions for excess
inventory quantities on hand. Any significant unanticipated changes in demand or unexpected quality failures could have a significant impact on the value of our inventory and reported operating results. During all periods presented in
the accompanying financial statements, there has been no material adjustments related to a revised estimate of inventory valuations.

 Jim B. Rosenberg

 April 5,
2007

  Page
 4

 Accrued Liabilities

 We prepare our financial statements in accordance with accounting principles generally accepted in the United States. These principles require that we
estimate accrued liabilities. This process involves identifying services, which have been performed on our behalf, and estimating the level of service performed and the associated cost incurred for such service as of each balance sheet date.
Examples of estimated accrued expenses include: 1) Fees paid to our contract manufacturers in conjunction with the production of clinical materials. These expenses are normally determined through a contract or purchase order
issued by the Company. 2) Service fees paid to organizations for their performance in conducting our clinical trials. These expenses are determined by contracts in place for those services and communications with project managers on costs which have
been incurred as of each reporting date. 3) Professional and consulting fees incurred with law firms, audit and accounting service providers and other third party consultants. These expenses are determined by either requesting those service
providers to estimate unbilled services at each reporting date for services incurred, or tracking costs incurred by service providers under fixed fee arrangements. We have processes in place to estimate the appropriate amounts to record for accrued
liabilities, which principally involve the applicable personnel reviewing the services provided. In the event that we do not identify certain costs which have begun to be incurred or we under or over-estimate the level of services performed
or the costs of such services, our reported expenses for that period may be too low or too high. The date on which certain services commence, the level of services performed on or before a given date, and the cost of such services are often
judgmental. We make these judgments based upon the facts and circumstances known to us at the date of the financial statements.

 A
change in the estimated cost or volume of services provided could result in additional accrued liabilities. Any significant unanticipated changes in such estimates could have a significant impact on our accrued liabilities and reported operating
results. There has been no material adjustments to our accrued liabilities in any of the periods presented in the accompanying financial statements.

 Results of Operations, page 22

2.
Please provide us in a disclosure-type format, revised discussions of your revenue fluctuations from period to period that clearly differentiate between volume and pricing
changes as required by Item 303(a)(3)(iii) of Regulation S-K.

 RESPONSE:

 We agree with the Staff’s recommendation that further discussion of the volume and pricing changes impacting our revenue fluctuations is appropriate. In response to
the Staff’s comment, we would have made the following revised disclosure as of March 31, 2006 and we are planning to include equivalent disclosure in our future filings on Form 10-K. We have included the entirety of the related disclosure
as of March 31, 2006 below for completeness. All proposed changes or additions to such disclosure to incorporate the Staff’s recommendations are highlighted in bold italic print.

 Jim B. Rosenberg

 April 5,
2007

  Page
 5

 Revenues

 Total revenue for fiscal 2006, 2005 and 2004 were $12,911,000, $9,360,000 and $6,914,000. Revenues for the years
ending March 31, 2006, 2005 and 2004 were primarily comprised of sales of our commercial products, Protein A and SecreFlo®. During the fiscal year ended March 31, 2006, 2005 and 2004 sales of our commercial products were:

Year ended March 31

% Change

2006

2005

2004

2006 vs. 2005

2005 vs. 2004

(in thousands, except percentages)

 Protein A

$
10,540

$
7,134

$
4,976

48
%

43
%

 SecreFlo®

1,989

2,189

1,867

-9
%

17
%

 Other product revenue

—

37

—

 Product revenue

$
12,529

$
9,360

$
6,843

34
%

37
%

 Substantially all of our products based on recombinant Protein A are sold to customers who incorporate our
manufactured products into their proprietary antibody purification systems to be sold directly to the pharmaceutical industry. Monoclonal antibodies are a well-established class of drug with applications in rheumatoid arthritis, asthma, Crohn’s
disease and a variety of cancers. Sales of Protein A are therefore impacted by the timing of large-scale production orders and on the regulatory approvals for such antibodies, which may result in significant quarterly fluctuations.

 During fiscal 2006, Protein A sales increased by $3,406,000 or 48%, primarily as a result of a rise in the demand for our Protein A products. The increase in sales
volume of Protein A resulted in an increase in revenues of 49%. This increase was slightly offset by a decrease in the total sales price of those products, which had an unfavorable impact of 1% on total revenues. During fiscal 2005, Protein A sales
increased by $2,158,000 or 43%, primarily as a result of a rise in the demand for our Protein A products. The increase in sales volume of Protein A resulted in an increase in revenues of 60%. This increase was partially offset by a decrease in the
total sales price of those products, which had an unfavorable impact of 17% on total revenues. The company sells different Protein A products at different price points. The mix of products sold varies and impacts the fluctuations in
total sales price from year to year. During the fourth quarter of fiscal 2005, a supply agreement with a key customer was amended to expand the scope of manufacturing and extend the term of the agreement through 2010 which resulted in
increased sales volume. During fiscal 2004, manufacturing problems experienced by one of our significant customers negatively impacted our sales of Protein A. We anticipate that sales of Protein A will continue to grow during the next year, but at a
reduced rate compared to the last two fiscal years and will continue to be subject to quarterly fluctuations due to timing of large-scale production orders.

 Jim B. Rosenberg

 April 5,
2007

  Page
 6

 Sales of SecreFlo® decreased $200,000 or 9% in fiscal 2006
primarily as a result of direct competition with our sole supplier of SecreFlo®. and as a
result of a reduction in sales and marketing efforts. To remain competitive we reduced sales prices, which had an unfavorable impact of 6% on SecreFlo® revenues and a decrease in volume of SecreFlo® vials sold, unfavorably impacting revenue by 3%.

 SecreFlo®
sales increased $322,000 or 17% in fiscal 2005 from $1,867,000 in fiscal 2004, primarily as a result of a delay during fiscal 2004 in the delivery of a new lot of
SecreFlo® from the manufacturer which negatively impacted 2004 revenue. A higher volume of
vials sold in 2005 increased revenues by 15% and an increase in sales prices increased revenues by an additional 2%. The settlement in fiscal 2005 with our sole supplier of SecreFlo® provides for a certain amount of vials of product that we can ultimately ship. The last shipment of Secreflo to the Company from
ChiRhoClin should be in late calendar year 2007 and will be sold to customers at least into fiscal year 2009.

3.
Although you identify the nature of the expense items associated with your fluctuations in operating expenses, you do not appear to discuss the underlying causes of those
changes. The following lists examples of your disclosure and is not intended to be exhaustive:

•

 higher clinical trial expenses of $192,000 (in your R&D disclosure);

•

 increased personnel costs of $124,000 (in your R&D disclosure);

•

 increased license expense of $59,000 (in your R&D disclosure);

•

 clinical material decreased by $124,000 (in your R&D disclosure);

•

 increased personnel expenses of $291

CORRESP
1
filename1.htm

SEC RESPONSE LETTER

 RepliGen

 Repligen Corporation

 41 Seyon
Street, Bldg #1

 Waltham, MA 02453-8335

 Telephone: 781-250-0111

 Telefax : 781-250-0115

                 March 13, 2007

 United States Securities and
Exchange Commission

 Division of Corporation Finance

 100 F
Street, N.E.

 Washington, D.C. 20549-7010

 Attention: Jim B.
Rosenberg / Mark Brunhofer

Re:
Repligen Corporation

Form 10-K for Fiscal Year Ended March 31, 2006 : File No. 0-14656

 Dear Gentlemen:

 This letter is being furnished on behalf of Repligen Corporation (the “Company”) in
response to comments in the letter dated March 5, 2007 (the “Letter”) from Jim B. Rosenberg of the Staff (the “Staff”) of the Securities and Exchange Commission (the “Commission”) to Walter C.
Herlihy, President and Chief Executive Officer of the Company, with respect to the Company’s Annual Report on Form 10-K for the fiscal year ended March 31, 2006 (the “Form 10-K”) that was filed with the Commission on
June 9, 2006.

 We respectfully request an extension of time until April 5, 2007 to provide our response. This will provide the time needed to
fully and adequately prepare and review the response with all applicable parties. Thank you for your consideration in this matter.

 If you require
additional information, please telephone the undersigned at (781) 419-1850.

 Sincerely,

 /s/ Daniel W. Muehl

 Daniel W. Muehl

 Chief Financial Officer

 Repligen Corporation

Via Facsimile and U.S. Mail
Mail Stop 6010
        March 5, 2007

Mr. Walter C. Herlihy
Chief Executive Officer and President
Repligen Corporation
41 Seyon Street
Building 1, Suite 100
Waltham, MA   02453

Re: Repligen Corporation
 Form 10-K for Fiscal Year Ended March 31, 2006
File No. 0-14656

Dear Mr. Herlihy:

We have reviewed your filing and have the following comments.  We have
limited our review to only your financial stat ements and related disclosures and do not
intend to expand our review to  other portions of your documents.  In our comments, we
ask you to provide us with more inform ation so we may better understand your
disclosure.  After reviewing this information, we may raise additional comments.

Please understand that the purpose of our re view process is to assist you in your
compliance with the applicable disclosure  requirements and to  enhance the overall
disclosure in your filings.  We look forward to working with you in these respects.  We
welcome any questions you may have about our comment or any other aspect of our
review.  Feel free to call us at the telephone numbers listed at the end of this letter.

Form 10-K for the fiscal year ended March 31, 2006

Management’s Discussion and Analysis of Fi nancial Condition and Results of Operations

Critical Accounting Estimates, page 20

1. Your revenue recognition disclosure does not appear to disclose the judgments
you must make each period and the uncertain ties surrounding those judgments.  In
addition, your revenue recognition, inventory reserve and accrued liabilities
disclosures do not appear to discuss the potential variability of reasonably likely
changes in your underlying estimates nor do your disclosures appear to discuss
how accurate your estimates have been in prior periods.  Please provide us in a
disclosure-type format, revised discussions  of your critical accounting estimates
that discuss specifically the judgmen ts you make, why your estimates or

Mr. Walter C. Herlihy
Repligen Corporation
March 5, 2007
Page 2
assumptions bear the risk of change, the impact on your financial results, financial
condition and liquidity of reasonably likely changes in the underlying
assumptions and the extent to which act ual subsequent experience has differed
materially from your initial estimates in each of the periods presented.  Please see
FR-72.

Results of Operations, page 22

2. Please provide us in a disclosure-type fo rmat, revised discussions of your revenue
fluctuations from period to period that clearly differentiate between volume and
pricing changes as required by Item  303(a)(3)(iii) of  Regulation S-K.

3. Although you identify the nature of the expense items associated with your
fluctuations in operating expenses, you do not appear to discuss the underlying
causes of those changes.  The following lis ts examples of your disclosure and is
not intended to be exhaustive:

• higher clinical trial expenses of $192,000 (in your R&D disclosure);
• increased personnel costs of $124,000 (in your R&D disclosure);
• increased license expense of $59,000 (in your R&D disclosure);
• clinical material decreased by $124,000 (in your R&D disclosure);
• increased personnel expenses of $291,000 (in your SG&A disclosure);
• increased professional expenses of $271,000 (in your SG&A disclosure); and
• increased legal expenses of $176,000 (in your SG&A disclosure).

Please provide us in a disclosure-type format revised operating expense discussions that clearly expl ain the underlying causes of all the various increase or
decreases you identify.  For example, pl ease explain why legal expenses increased
by $176,000 in fiscal 2006.

Financial Statements

Note 3:  Long-Lived Assets, page F-13

4. You disclose that you recorded an impairm ent charge in fiscal 2004 related to a
license intangible with ChiRhoClin, Inc. re sulting from a disput e with the licensor
and that you fully amortized the remain ing intangible asset in fiscal 2005.  In
Note 11 on page F-18, in Business on page 8 and in Legal Proceedings on page 17, you disclose that you entered into a se ttlement agreement with ChiRhoClin in
May 2005 whereby you are entitled to continue  to market SecreFlo for the next
several years and ChinRhoClin will be obligated to provide you supplies of
SecreFlo for the next few years.  Please explain to us how you determined your impairment and why you have no remain ing intangible asset at March 31, 2005
when you apparently continue to sell Secr eFlo.  In your response, please explain
to us why you do not disclose the exact or estimated date through which you may
market SecreFlo and an exact or estim ated date through which you expect to

Mr. Walter C. Herlihy
Repligen Corporation
March 5, 2007
Page 3
purchase SecreFlo from ChiRhoClin.  In addition, please expl ain to us why you
do not apparently disclose and discuss your SecreFlo revenues in your fiscal 2007
Forms 10-Q.

Please respond to these comments within  10 business days or tell us when you
will provide us with a response.  Please s ubmit a letter that keys your responses to our
comments and provides the requested information.  Detailed letters greatly facilitate our
review.  Please submit your letter to us via EDGAR under the form type label
CORRESP.

We urge all persons who are responsible for the accuracy and adequacy of the
disclosure in the filing to be certain that the filing includes all in formation required under
the Securities Exchange Act of 1934 and th at they have provided all information
investors require for an informed invest ment decision.  Since the company and its
management are in possession of all facts re lating to a company’s disclosure, they are
responsible for the accuracy and adequacy of the disclosures they have made.

 In connection with responding to our co mments, please provide, in your letter, a
statement from the company acknowledging that:

‚ the company is responsible for the adequacy  and accuracy of the disclosure in the
filing;

‚ staff comments or changes to disclosure  in response to staff comments do not
foreclose the Commission from taking any action with respect to the filing; and

‚ the company may not assert staff comments as a defense in any proceeding initiated
by the Commission or any person under the federal securities laws of the United
States.

In addition, please be advise d that the Division of Enfo rcement has access to all
information you provide to the staff of the Divi sion of Corporation Fi nance in our review
of your filing or in response to our comments on your filing.

If you have any questions, please co ntact Mark Brunhofer , Senior Staff
Accountant, at (202) 551-3638.  In  this regard, do not hesita te to contact me, at (202)
551-3679.

Sincerely,

Jim B. Rosenberg
Senior Assistant Chief Accountant