SecProbe.io

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 December 9, 2025
 Via EDGAR U.S. Securities and Exchange Commission
 Division of Corporation Finance 100 F Street, N.E.
 Washington, D.C. 20549

 Attn:
 Tim Buchmiller

 Re:
 REGENXBIO Inc. (the “Company”)
 Registration Statement Filed on Form S-3 (the “Registration Statement”)
 File No. 333-291816 Ladies and
Gentlemen: Pursuant to Rule 461 under the Securities Act of 1933, as amended, the Company hereby respectfully requests that the effective
date for the above-referenced Registration Statement be accelerated to 4:00 p.m., Eastern Time, on December 12, 2025, or as soon as practicable thereafter.
 Please contact Mathew C. Franker of Covington & Burling LLP at (202) 662-5895 or
mfranker@cov.com with any questions you may have regarding this request. In addition, please notify Mr. Franker by telephone when this request for acceleration has been granted.

 Respectfully,

 REGENXBIO Inc.

 By:

 /s/ Patrick Christmas

 Name:

 Patrick Christmas

 Title:

 Executive Vice President, Chief Legal
Officer

 cc:
 Mathew C. Franker, Covington & Burling LLP

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 June 23, 2025
 Via EDGAR U.S. Securities and Exchange Commission
 Division of Corporation Finance 100 F Street, N.E.
 Washington, D.C. 20549

 Attn:
 Doris Stacey Gama

 Re:
 REGENXBIO Inc. (the “Company”)

  
 Registration Statement Filed on Form S-3 (the “Registration
Statement”)

  
 File No. 333-288053
 Ladies and Gentlemen: Pursuant to
Rule 461 under the Securities Act of 1933, as amended, the Company hereby respectfully requests that the effective date for the above-referenced Registration Statement be accelerated to 4:00 p.m., Eastern Time, on June 25, 2025, or as
soon as practicable thereafter. Please contact Matthew C. Franker of Covington & Burling LLP at
 (202) 662-5895 or mfranker@cov.com with any questions you may have regarding this request. In addition, please notify Mr. Franker by telephone when this request for acceleration has been granted.

 Respectfully,

 REGENXBIO Inc.

 By:

 /s/ Patrick Christmas

 Name:

 Patrick Christmas

 Title:

 Executive Vice President, Chief Legal
Officer

 cc:
 Matthew C. Franker, Covington & Burling LLP

<DOCUMENT>
<TYPE>TEXT-EXTRACT
<SEQUENCE>2
<FILENAME>filename2.txt
<TEXT>
 June 20, 2025

Curran Simpson
Chief Executive Officer
REGENXBIO Inc.
9804 Medical Center Drive
Rockville, MD 20850

 Re: REGENXBIO Inc.
 Registration Statement on Form S-3
 Filed June 13, 2025
 File No. 333-288053
Dear Curran Simpson:

 This is to advise you that we have not reviewed and will not review your
registration
statement.

 Please refer to Rules 460 and 461 regarding requests for acceleration.
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 Doris Stacey Gama at 202-551-3188 with any questions.

 Sincerely,

 Division of
Corporation Finance
 Office of Life
Sciences
cc: Matthew C. Franker, Esq.
</TEXT>
</DOCUMENT>

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 REGENXBIO Inc.

9600 Blackwell Road, Suite 210

Rockville, MD 20850

 January 4, 2017

VIA EDGAR

 Securities and Exchange Commission

 Division of Corporation Finance

 100 F. Street, N.E.

Washington, D.C. 20549

 Attention: Mary Beth Breslin

Re:
REGENXBIO Inc.

 Registration Statement on Form
S-3 (File No. 333-215146)

 Dear Ms. Breslin:

Pursuant to Rule 461 under the Securities Act of 1933, as amended, REGENXBIO Inc. (the “Company”) hereby requests that the Securities and Exchange
Commission (the “Commission”) take appropriate action to make the above-referenced Registration Statement on Form S-3 effective at 5:00 p.m. Eastern Time on Friday, January 6, 2017 or as soon
thereafter as practicable.

 The Company hereby authorizes Albert Vanderlaan of Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, LLP,
counsel to the Company, to orally modify or withdraw this request for acceleration.

 Please provide a copy of the Commission’s order declaring the
Registration Statement effective to Albert Vanderlaan at Gunderson Dettmer, One Marina Park Drive, Suite 900, Boston, MA 02210. If possible, please also send such order by email to the attention of Albert Vanderlaan at avanderlaan@gunder.com.

Thank you for your attention on this matter.

Very truly yours,

REGENXBIO Inc.

By:

 /s/ Kenneth T. Mills

Kenneth T. Mills

President and Chief Executive Officer

cc:
Patrick Christmas, REGENXBIO Inc.

 Jay Hachigian, Gunderson Dettmer Stough Villeneuve
Franklin & Hachigian, LLP

 Keith Scherer, Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, LLP

Albert Vanderlaan, Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, LLP

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 REGENXBIO INC.

9712 MEDICAL CENTER DRIVE, SUITE 100

ROCKVILLE, MD 20850

September 14, 2015

 VIA EDGAR

Securities and Exchange Commission

 Division of Corporation
Finance

 100 F. Street, N.E.

 Washington, D.C. 20549

Attention:
Jeffrey P. Riedler

 Preston Brewer

John Krug

Re:
REGENXBIO Inc.

 Registration Statement on Form S-1 (File No. 333-206430)

 Request for Acceleration of Effective Date

Dear Mr. Riedler:

 Pursuant to Rule 461
under the Securities Act of 1933, as amended, REGENXBIO Inc. (the “Registrant”) hereby requests acceleration of the effective date of the above-referenced Registration Statement on Form S-1 (File No. 333-206430) (as amended,
the “Registration Statement”), so that it may become effective at 5:00 p.m. Eastern Time on September 16, 2015, or as soon thereafter as practicable. By separate letter, the representatives of the underwriters of the issuance
of the securities being registered join this request for acceleration.

 The Registrant hereby acknowledges that:

(i) should the Securities and Exchange Commission (the “Commission”) or the staff, acting pursuant to delegated authority,
declare the Registration Statement effective, it does not foreclose the Commission from taking any action with respect to the Registration Statement;

(ii) the action of the Commission or the staff, acting pursuant to delegated authority, in declaring the Registration Statement effective,
does not relieve the Registrant from its full responsibility for the adequacy and accuracy of the disclosure in the Registration Statement; and

(iii) the Registrant may not assert staff comments and the declaration of effectiveness as a defense in any proceeding initiated by the
Commission or any person under the federal securities laws of the United States.

 [Signature Page Follows]

 1

 Please do not hesitate to contact our legal counsel, Albert Vanderlaan at (617) 648-9298 if
you have any questions or would like additional information regarding this matter.

Sincerely,

REGENXBIO Inc.

By:

 /s/ Kenneth T. Mills

Kenneth T. Mills

President and Chief Executive Officer

cc:

Jay K. Hachigian

Richard R. Hesp, Esq.

Keith J. Scherer, Esq.

Albert W. Vanderlaan, Esq.

Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, LLP

 2

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 Morgan Stanley & Co. LLC

1585 Broadway

 New York, NY 10036

Merrill Lynch, Pierce, Fenner & Smith

                     Incorporated

One Bryant Park

 New York, NY 10036

September 14, 2015

 VIA EDGAR

U.S. Securities and Exchange Commission

 Division of Corporation
Finance

 100 F Street, N.E.

 Washington, DC 20549

Re: REGENXBIO Inc.

       Registration Statement, as amended on Form S-1 (File No. 333-206430)

Ladies and Gentlemen:

 In accordance with the
above-referenced Registration Statement, and pursuant to Rule 461 of the General Rules and Regulations of the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Securities Act”), we, the representative of
the several underwriters (the “Representative”), hereby join in the request of REGENXBIO Inc. that the effective time of the above-referenced Registration Statement be accelerated so that it will be declared effective at 5:00 p.m., Eastern
time, on September 16, 2015, or as soon thereafter as practicable.

 Pursuant to Rule 460 under the Securities Act, please be advised
that during the period from September 8, 2015 to the date of this letter, the preliminary prospectus, dated September 8, 2015, in connection with the Registration Statement was distributed approximately as follows:

 Copies to underwriters:

487

 Copies to prospective institutional investors:

1,925

 Copies to prospective retail investors and others:

666

 Total:

3,078

 The Representative confirms on behalf of itself and the several underwriters that they have complied and will
continue to comply with the requirements of Rule 15c2-8 promulgated under of the Securities Exchange Act of 1934, as amended, in connection with the above-referenced issue.

Very truly yours,

 MORGAN STANLEY & CO. LLC

MERRILL LYNCH, PIERCE, FENNER & SMITH

    INCORPORATED

As Representative of the several Underwriters

By:

MORGAN STANLEY & CO. LLC

By:

 /s/ David Lederman

Name:

David Lederman

Title:

Vice President

By:

 MERRILL LYNCH, PIERCE, FENNER & SMITH

    INCORPORATED

By:

 /s/ Richard A. Diaz

Name:

Richard A. Diaz

Title:

Authorized Signatory

 [Signature Page to Representative Acceleration Request]

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CORRESP

 August 28, 2015

VIA EDGAR AND COURIER

 Securities and Exchange
Commission

 Division of Corporation Finance

 100 F. Street,
N.E.

 Washington, D.C. 20549

Attention:

Jeffrey P. Riedler

Preston Brewer

John Krug

Re:
REGENXBIO Inc.

Registration Statement on Form S-1

Filed August 17, 2015

File No. 333-206430

Amendment No. 1 to Draft Registration Statement on Form S-1

Submitted August 10, 2015

CIK No. 0001590877

 Dear Messrs. Riedler, Brewer and Krug:

On behalf of REGENXBIO Inc. (the “Company”), we submit this letter in response to comments from the staff (the “Staff”) of
the Securities and Exchange Commission (the “Commission”) received by letter dated August 26, 2015 relating to the Company’s Registration Statement on Form S-1 filed on August 17, 2015 (the “Registration
Statement”) and originally submitted as a draft registration statement on Form S-1 on July 1, 2015.

 For the convenience of the
Staff, we are providing to the Staff by courier copies of this letter.

 In this letter, we have recited the written comments from the
Staff in italicized, bold type and have followed each comment with the Company’s response.

 August 28, 2015

Page
 2

 8. Stock-based Compensation, page
F-33

1.
With regard to your response to comment 24 please tell us how you considered the following in determining expected volatility for stock-based compensation in accordance with ASC 718:

•

Using BioMarin and Regeneron as similar when they have significantly greater market capitalizations, revenues, and research and development expense than your company,

•

Using the volatility of companies with less than six years of trading data (like Spark from only February 2015 to May 2015) to estimate expected volatility over the six year expected term of the options,
and

•

Tell us how you intend to estimate expected volatility for future stock option grants.

RESPONSE TO COMMENT 1:

Selection of BioMarin and Regeneron as peer group companies

The Company respectfully advises the Staff that its selection of peer group companies for determining expected volatility is weighted toward
companies in its industry, and more specifically with similar disease focuses and technologies. Additionally, the Company considers the size, stage of life-cycle and financial leverage of potential peer group companies relative to the
Company’s, as well as the availability of historical trading data when developing an estimate for the expected volatility over the expected term of the underlying stock options. The Company’s selection of BioMarin Pharmaceuticals, Inc.
(“BioMarin”) and Regeneron Pharmaceuticals, Inc. (“Regeneron”) as peer group companies was based on the similarity of those companies’ disease focus and technology to those of the Company, their financial leverage and the
availability of active trading history. The following factors were considered when assessing the similarity of BioMarin and Regeneron to the Company and the appropriateness for their inclusion within the Company’s list of peer group companies:

•

BioMarin. BioMarin focuses on treatments for a number of rare diseases that are also in the Company’s development pipeline or in the pipelines of the Company’s licensees, including treatments for MPS I
(approved), PKU (approved), Pompe Disease (Phase III) and hemophilia A (preclinical). Additionally, BioMarin’s preclinical candidate for hemophilia A uses AAV technology to deliver genetic material to the body that is similar to the technology
used by the Company and its licensees.

•

 Regeneron. Regeneron has an approved product for the treatment of wet AMD, which is a disease that is also in the Company’s lead product
development pipeline. Additionally, Regeneron has an approved product which regulates LDL-C levels for patients with HeFH, a similar disease to HoFH (in which patients suffer from abnormally high LDL levels), a disease that is in the Company’s
lead

 2

 August 28, 2015

Page
 3

product development pipeline. In May 2014, Regeneron and Avalanche Biotechnologies, Inc. (“Avalanche”), a gene therapy company using technology similar to that of the Company, entered into a collaboration agreement to
develop and commercialize novel gene therapy products for the treatment of ophthalmologic diseases using AAV technology.

The Company also notes that BioMarin and Regeneron each have available trading histories to estimate volatility over the expected term of the
Company’s stock option awards. The Company recognizes that BioMarin and Regeneron are larger than the Company in terms of market capitalization and are at a later stage of development than the Company. However, the Company believes that the
expected volatility of the Company’s common stock is more similar to these companies’ historical volatility than other biotechnology companies of similar size which lack similarity to the Company in terms of disease focus and underlying
technology as it relates to the use of AAV-based gene therapies. BioMarin and Regeneron were included in the Company’s peer group selection due to these similarities, in combination with a sufficient trading history which is generally not
present in other companies with similar focus and technology as the Company.

 Selection of companies with available trading history
less than the expected term of the Company’s stock option awards as peer group companies

 As stated above, the Company focuses
the selection of its peer group companies on companies with similar disease focus and technology, specifically as it relates to gene therapy using AAV technology. The Company believes the historical volatility of these companies is the best
indicator of the Company’s expected volatility because stock price changes for these companies are primarily driven by investors’ and research analysts’ opinions on the safety and efficacy of the underlying technology. Accordingly,
the Company believes that the AAV-based gene therapy companies used in its peer group, including bluebird bio, Inc., Applied Genetic Technologies Corporation, Spark Therapeutics, Inc. and Avalanche, are appropriate. However, the Company acknowledges
that gene therapy technology is only recently receiving significant attention from the investment community. As such, none of these companies were publicly-traded prior to 2013, and accordingly these companies’ have less historical trading data
than the expected term of the Company’s stock option awards. The Company does not believe that peer group companies’ history of trading data must be at least equal to the expected term of the Company’s stock options in order to
provide a meaningful estimate of the Company’s common stock volatility. However, in order to account for the lack of long-term trading history for these companies, the Company also included companies like BioMarin, Regeneron and Sangamo
Biosciences, Inc. in its peer group selection, which have long-term historical trading data available in excess of the expected term of stock options awarded.

Impact of peer group company selections on expected volatility

The Company performed a sensitivity analysis on its expected volatility for the year ended December 31, 2014 and the six months ended
June 30, 2015 to determine whether placing a higher focus on peer group companies more similar to the size of the Company, and with

 3

 August 28, 2015

Page
 4

 available trading data at least equal
to the expected term of the Company’s stock option awards to employees, would have a material effect on volatility and stock-based compensation expense in the Company’s financial statements. In the analysis, the Company replaced its peer
group company selections with biotechnology companies which are all similar to the Company in terms of disease indication (although using different technologies), size, stage of development and financial leverage, and which have an available trading
history in excess of six years. As a result of the analysis, the Company determined that the alternative peer group selections, and resulting expected volatility, had an immaterial effect on the Company’s financial statements in total and for
any periods presented in the Registration Statement. Accordingly, the Company determined that its peer group company selection provided a reasonable basis for expected volatility for the year ended December 31, 2014 and the six months ended
June 30, 2015.

 Estimating expected volatility for future stock option grants

The Company respectfully advises the Staff that until it has sufficient trading history, it plans to continue to use historical volatility of
peer group companies to estimate expected volatility of its common stock. The Company will place a higher focus on companies with market capitalization and stage of development more similar to the Company’s, and that have a longer history of
available trading data closer to the expected term of the Company’s stock option grants. Assuming the consummation of the Company’s proposed initial public offering, the Company will use a hybrid approach to estimate expected volatility
and therefore will include its own common stock volatility along with the volatility of the peer group companies. The Company will initially place a low weight on its own common stock volatility, and will increase that weighting as more historical
trading data becomes available over time.

2.
Refer to our prior comment 25. You disclose on page 166 that certain of your directors and officers purchased an aggregate of 6.95 million shares of Dimension common stock for an aggregate price of $695.45.
Please explain why the use of the OPM method based on a preferred stock issuance was appropriate to value Dimension’s common stock when there was a contemporaneous issuance of its common stock.

RESPONSE TO COMMENT 2:

The Company respectfully advises the Staff that the Company’s directors and officers purchased 6,954,536 shares of Dimension Therapeutics,
Inc. (“Dimension”) common stock at a purchase price equal to the par value of $0.0001 per share, for an aggregate purchase price of $695.45. The Company notes that the 6,954,536 shares was part of a total purchase of 9,990,000 shares of
Dimension common stock by directors, officers and members of the Company, as disclosed in page F-42 of the Registration Statement. The purchase was in connection with a license agreement issued by the Company to Dimension, and no other shares of
Dimension were issued for cash proceeds on the date of this transaction. Due to its related party nature, the Company does not view the purchase of the Dimension common stock, at par value, to be a contemporaneous sale of common stock for which a
fair value can be relied upon.

 On the same date, Dimension issued preferred stock to an unrelated investor. The Company applied the
back-solve method of the option-pricing method (“OPM”) using the per share price of the preferred stock issuance, which indicated that the fair value of Dimension’s common stock exceeded its par value on the date it was purchased.
Since the preferred stock was issued to an unrelated party, the Company believes the fair value of Dimension common stock under the OPM is a more reliable indicator of fair value than the par value per share paid by the directors and officers of the
Company.

 4

 August 28, 2015

Page
 5

3.
Please confirm for us whether you received 10,000 shares of Dimension’s common stock or 10 million shares and how you determined the amount of revenue to recognize. You recognized $2.7 million of revenue
which would represent 10 million shares at $0.27 per share. However, Exhibit 10.18 states that the consideration was 10,000 shares which would be $2,700 at $0.27 per share.

RESPONSE TO COMMENT 3:

The Company respectfully advises the Staff that, in accordance with the agreement filed as Exhibit 10.18 to the Registration Statement, the
Company received 10,000 shares of Dimension’s common stock as consideration for the license granted by the Company. However, simultaneous with the issuance of the 10,000 shares to the Company, Dimension sold and issued an additional 9,990,000
shares of its common stock to directors, officers and members of the Company (the “Related Parties”) at par value of $0.0001 per share, which were estimated by the Company to have a fair value of $0.27 per share.

The Company acknowledges that, in form, 9,990,000 shares of Dimension common stock were sold directly to the Related Parties by Dimension at
par value, and 10,000 shares of Dimension common stock were issued to the Company in consideration of the license granted to Dimension; however, due to its related party nature, the Company determined that for the purposes of its financial
statements, in substance, the transaction is deemed to be an intellectual property license granted by the Company to Dimension, for a total deemed consideration of 10,000,000 shares of Dimension common stock, 9,990,000 of which were then deemed
granted to the Related Parties by the Company as compensation. The only consideration transferred to Dimension in the transaction (excluding the nominal par value per share paid by the Related Parties) was the intellectual property license granted
by the Company. As disclosed on page F-42 of the Registration Statement, the Company recorded license revenue related to the license grant in accordance with the deemed nature of the transaction, which is equal to 10,000,000 shares of Dimension
common stock at a fair value of $0.27 per share. As disclosed on page F-43 of the Registration Statement, the Company recorded compensation expense to the Related Parties equal to the fair value of the 9,990,000 shares at $0.27 per share, for a
total of $2,697,300. The 10,000 shares of Dimension common stock issued to the Company as a result of the license agreement were recorded on the Company’s balance sheet as a cost method investment.

[Remainder of page intentionally left blank.]

 5

 August 28, 2015

Page
 6

*    *    *    *    *

Please do not hesitate to contact me at (617) 648-9101, Richard Hesp at (617) 648-9230 or Keith Scherer at (617) 648-9231 if
you have any questions or would like additional information regarding this matter.

 Very truly yours,

GUNDERSON DETTMER STOUGH

 VILLENEUVE FRANKLIN &
HACHIGIAN LLP

By:

 /s/ Jay K. Hachigian

cc:

Kenneth T. Mills

Vittal Vasista

Sara Garon Berl

REGENXBIO Inc.

Richard Hesp

Keith Scherer

Albert Vanderlaan

Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, LLP

Richard D. Truesdell, Jr.

Jeffrey Ramsay

Davis Polk and Wardwell LLP

 6

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SUPLTR

 AUGUST 27, 2015

CONFIDENTIAL TREATMENT REQUESTED BY REGENXBIO INC.

PORTIONS OF THIS DOCUMENT HAVE BEEN OMITTED PURSUANT TO A REQUEST

FOR CONFIDENTIAL TREATMENT AND, WHERE APPLICABLE, HAVE BEEN MARKED

WITH AN ASTERISK TO DENOTE WHERE OMISSIONS HAVE BEEN MADE. THE

CONFIDENTIAL MATERIAL HAS BEEN FILED SEPARATELY WITH THE SECURITIES

AND EXCHANGE COMMISSION.

 Via EDGAR and
Courier

 U.S. Securities and Exchange Commission

 Division
of Corporation Finance

 100 F Street, N.E.

 Washington, D.C.
20549

Attention:

Jeffrey P. Riedler

Preston Brewer

John Krug

Re:

REGENXBIO Inc.

Registration Statement on Form S-1 (File No. 333-206430)

Filed August 17, 2015

 Dear Mr. Riedler:

 On
behalf of REGENXBIO Inc. (the “Company”), and in connection with the submission of a letter dated August 10, 2015 (the “Response Letter”) in response to comments from the staff (the “Staff”) of
the Securities and Exchange Commission (the “Commission”) contained in your letter dated July 28, 2015 (the “Comment Letter”), relating to the above-referenced Registration Statement on Form S-1 originally
confidentially submitted by the Company with the Commission on July 1, 2015 and subsequently filed with the Commission on August 17, 2015 (the “Registration Statement”), we submit this supplemental letter to further
address comment 11 of the Comment Letter.

 Because of the commercially sensitive nature of information contained herein, this submission is accompanied by
a request for confidential treatment for selected portions of this letter. The Company has filed a separate letter with the Office of Freedom of Information and Privacy Act Operations in connection with the confidential treatment request, pursuant
to Rule 83 of the Commission’s Rules on Information and Requests, 17 C.F.R. § 200.83. For the Staff’s reference, we have enclosed a copy of the Company’s letter to the Office of Freedom of Information and Privacy Act Operations,
as well as a copy of this correspondence, marked to show the portions redacted from the version filed via EDGAR and for which the Company is requesting confidential treatment. For the convenience of the Staff, we are providing to the Staff copies of
this letter by courier.

 In this letter, we have recited comment 11 of the Comment Letter in italicized, bold type and have followed the comment with the
Company’s response. Except as otherwise specifically indicated, page references herein correspond to the page of the Registration Statement, as applicable.

 Securities and Exchange Commission

August 27, 2015

  Page
 2

 Critical Accounting Policies and
Significant Judgments and Estimates

 Stock-Based Compensation

Common Stock Valuation Methodology, page 77

11.
We may have additional comments on your accounting for equity issuances including stock compensation and beneficial conversion features. Once you have an estimated offering price, please provide us an analysis
explaining the reasons for the differences between recent valuations of your common stock leading up to the IPO and the estimated offering price.

To assist the Staff in its evaluation of the Company’s stock-based compensation, the Company has provided the analysis as set forth in this letter.

The Company will set forth a bona fide initial public offering (“IPO”) price range in a pre-effective amendment to the Registration Statement
prior to the distribution of any preliminary prospectus, please see “Discussion of Estimated Offering Price” below.

 GRANTS OF COMMON STOCK
OPTIONS IN THE PRECEDING 12 MONTHS

 The following table summarizes by grant date the number of shares of common stock underlying stock options granted
during the previous 12 months, as well as the associated per share exercise price and the estimated fair value per share of the Company’s common stock on the grant date to determine stock based compensation expense for financial reporting
purposes:

 Grant Date

Number of
Common Shares
Underlying Options
Granted

Exercise Price Per
Common Share

Estimated Fair
Value Per Common
Share (a)(b)

 September 24, 2014

1,884,500

$
0.85

$
0.52

 November 4, 2014

247,900

$
0.85

$
0.49

 May 19, 2015

1,063,900

$
3.76

$
2.27

(a)
The Estimated Fair Value per Common Share reflects the weighted-average fair value per share of common stock of options granted on each grant date, determined using the Black-Scholes option-pricing model.

(b)
For purposes of recording stock-based compensation for grants of options to non-employees, the Company measures the fair value of the award on the service completion date (vesting date). At the end of each reporting
period prior to completion of the services, the Company remeasures the value of any unvested portion of the option based on the then-current fair value of the option and adjusts the expense accordingly. The weighted-average fair value amounts
presented in this column for grants to non-employees reflect only the grant-date fair value of options granted to non-employees and not any subsequently remeasured fair value of those options.

DETERMINATION OF FAIR VALUE OF COMMON STOCK

 The Company
respectfully directs the Staff to pages 76 through 82 of the Registration Statement for a thorough discussion of the Company’s methodology for determining the fair value of its common stock. As described in the Registration Statement, the
Company has historically determined the fair value of its

 ***
Confidential material redacted

 CONFIDENTIAL TREATMENT REQUESTED BY REGENXBIO INC.

 Securities and Exchange Commission

August 27, 2015

  Page
 3

 common stock using methodologies,
approaches and assumptions consistent with the American Institute of Certified Public Accountants Accounting and Valuation Guide, Valuation of Privately-Held-Company Equity Securities Issued as Compensation (the “Practice Guide”).
In addition, the Company’s board of directors (the “Board”) also considered numerous objective and subjective factors, along with input from management, to determine the fair value of the Company’s common stock as
disclosed in the Registration Statement.

 The Company’s common stock valuations were performed using the market multiple approach, option-pricing
method (“OPM”) back-solve method or a hybrid of the probability-weighted expected return method (“PWERM”) and the OPM, which the Company refers to as the “Hybrid Method”. Each of the methods is
described as follows:

•

Market multiple approach. The market multiple approach estimates the fair value of a company by applying market multiples of comparable publicly-traded companies and publicly disclosed financial data to arrive at
an estimated equity value. That equity value is then allocated to the various classes of equity using the OPM to arrive at the fair value of common stock.

•

OPM. The OPM back-solve method derives the implied equity value for one type of equity security from a contemporaneous transaction involving another type of equity security. The OPM treats common stock and
convertible preferred stock as call options on an equity value, with exercise prices based on the liquidation preference of the convertible preferred stock. Therefore, the common stock has value only if the funds available for distribution to the
stockholders exceed the value of the liquidation preference at the time of a liquidity event such as a merger, sale or IPO, assuming the enterprise has funds available to make a liquidation preference meaningful and collectible by the stockholders.
The common stock is modeled to be a call option with a claim on the enterprise at an exercise price equal to the remaining value immediately after the convertible preferred stock is liquidated. The OPM uses the Black-Scholes option-pricing model to
price the call options. The OPM is appropriate to use when the range of possible future outcomes is so difficult to predict that forecasts would be highly speculative.

•

PWERM. The PWERM is a scenario-based analysis that estimates the per share value of common stock based on the probability-weighted present value of expected future equity values for the common stock, under
various possible future liquidity event scenarios, in light of the rights and preferences of each class of stock, discounted for time to liquidity event at a rate that considers lack of marketability. The probability weighting of each potential
liquidity event used in its PWERM analysis is based on the Company’s stage of development, the status of its research and development efforts and financial position, external market conditions affecting the biotechnology industries, the
volatility in the capital markets, especially with respect to initial public offerings, and the relative likelihood of achieving a liquidity event such as an initial public offering or sale of the Company in light of prevailing market conditions.

 VALUATIONS AND OPTION GRANTS OVER THE PREVIOUS 12 MONTHS

The Company respectfully directs the Staff to pages 78 through 82 of the Registration Statement for a thorough discussion of the Company’s valuation and
option grants over the previous 12 months. The Company performed three valuations of its common stock as of the following dates: July 31, 2014 (the “July 2014 Valuation Date”), April 30, 2015 (the “April 2015
Valuation Date”) and June 15, 2015 (the “June 2015 Valuation Date”).

*** Confidential material redacted

CONFIDENTIAL TREATMENT REQUESTED BY REGENXBIO INC.

 Securities and Exchange Commission

August 27, 2015

  Page
 4

 July 2014 Valuation Price

At the July 2014 Valuation Date, the fair value of the Company’s common stock was estimated to be $0.85 per share (the “July 2014 Valuation
Price”). After due consideration, the Board determined the fair market value of the Company’s common stock as of the September 24, 2014 and November 4, 2014 stock option grant dates was equal to the July 2014 Valuation Price.
The Company believes there were no significant events that occurred between the July 2014 Valuation Date and the September 24, 2014 and November 2, 2014 stock option grant dates which had a significant effect on the Company’s equity
value. Specifically, there were no significant changes in the Company’s business, including its financing, key management personnel, intellectual property licensing, technology or research and development, that would suggest a change in the
fair value of the Company’s common stock from the July 2014 Valuation Date. The July 2014 Valuation Price was determined using a hybrid of the market multiple approach (20% weighting) and OPM back-solve method (80% weighting).

Under the market multiple approach, the Company applied a market multiple of revenue of comparable publicly-traded companies to its estimated revenue for the
year ended December 31, 2014 to arrive at an estimated equity value. Consideration was given to differences between the Company and the selected guideline public companies in terms of size, anticipated profitability, market size and other
critical characteristics that generally reflect an investor’s assessment of the business and financial risks inherent in the industry. In particular, the Company gave consideration to the fact that it had no approved products or clinical-stage
product candidates in its development pipeline at such time compared to comparable publicly-traded companies that had clinically-advanced pipelines and approved drug products.

Under the OPM, the Company applied the OPM back-solve method to solve for the equity value, and corresponding value of common stock, based on the price per
unit of its Series B Preferred Units issued and sold in October 2013. The Series B Preferred Units financing was led by a third-party investor that had not previously invested in the Company. The Company believes there were no significant events in
the Company’s business that occurred between October 2013 and the July 2014 Valuation Date which had a significant effect on the value of the Series B Preferred Units (and corresponding value of common stock). Accordingly, the Company believes
the per unit issuance price of the Series B Preferred Units provides an indication of the fair value of the Company’s equity as of the July 2014 Valuation Date.

The OPM is appropriate to use when the range of possible future outcomes is so difficult to predict that forecasts, and other potential outcomes, would be
highly speculative. The Company believes that a weighting of 80% on the OPM back-solve method was appropriate given the high amount of speculation required to predict future outcomes and forecasts due to the Company’s early stage of
development, lack of capitalization and key management personnel and pre-clinical stage of development of any potential product candidates at the date of the valuation. In order to capture the effects of market forces on the equity value of the
business, the Company deemed it appropriate to apply a 20% weighting on the market multiple approach. A higher weighting was placed on the OPM over the market multiple approach because at such time the Company did not have any approved product sales
and revenues were primarily driven by license revenue which is highly unpredictable and may fluctuate significantly from period to period given its contingent nature. The Company determined that as of the July 2014 Valuation Date, the market
multiple approach and OPM back-solve methods were more appropriate than a PWERM approach due to the high level of speculation that would be required to predict future liquidity events such as a sale, merger or IPO. Specifically, due to the
Company’s limited capitalization and early stage of research and development for it potential lead product candidates, the Company believed that such liquidity events would be highly contingent upon multiple additional successful private
financing rounds from new investors, the hiring of key management personnel and the advancement of its lead product candidates into clinical trials. Given the uncertainty surrounding those events, a PWERM approach was not considered a reliable
indicator of fair value.

 *** Confidential material redacted

CONFIDENTIAL TREATMENT REQUESTED BY REGENXBIO INC.

 Securities and Exchange Commission

August 27, 2015

  Page
 5

 April 2015 Valuation Price

 At the April 2015 Valuation Date, the fair value of the Company’s common stock was estimated to be $3.76 per share (the “April 2015
Valuation Price”). After due consideration, the Board determined the fair market value of the Company’s common stock as of the May 19, 2015 stock option grant date was equal to the April 2015 Valuation Price. The Company believes
there were no significant events that occurred between the April 2015 Valuation Date and the May 19, 2015 stock option grant date which had a significant effect on the Company’s equity value, with the exception of the issuance and sale of
Series D Preferred Stock on May 15, 2015, the terms of which were incorporated into the April 2015 Valuation Price. The April 2015 Valuation Price was determined using the Hybrid Method, and applied a 15% probability-weighting to the PWERM and
an 85% probability-weighting to the OPM.

 The PWERM utilized to determine the April 2015 Valuation Price assumed an IPO date five months from the April
2015 Valuation Date. The PWERM considers two possible outcomes: (i) a future equity value upon an IPO at the higher end of an estimated range and (ii) a future equity value upon an IPO at the lower end of an estimated range. In order to
estimate the range of potential future equity values upon an IPO for the PWERM, the Company considered the pre-money enterprise values at the IPO date of comparable companies that had undergone IPOs in recent periods prior to the April 2015
Valuation Date. The Company placed a 35% probability-weighting on the higher end of the range of expected future equity values and a 65% probability-weighting on the lower end of the range, based on the stage of developmen

Mail Stop 4720

August 26 , 2015

Via E -Mail
Kenneth T. Mills
Chief Executive Officer
REGENXBIO Inc.
9712 Medical Center Drive, Suite 100
Rockville, MD 20850

Re: REGENXBIO Inc.
Registration Statement on Form S -1
Filed August 17, 2015
  File No. 333-206430

Amendment No. 1 to Draft Registration Statement on Form S -1
Submitted August 10, 2015
  CIK No. 0001590877

Dear Mr. Mills:

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

Please respond to this letter by amending your registration statement and providing the
requested information.   If you do not believe our comments apply to your facts and
circumstances or do not believe an amendment is appropriate, please tell us why in yo ur
response.

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

8. Stock -based Compensation, page F -33

1. With regard to your response to comment 24 please tell us how you considered the
following in determining expected volatility for stock -based compensation in accordance
with ASC 718:

Kenneth T. Mills
REGENXBIO Inc.
August 26 , 2015
Page 2

  Using BioMarin and Regeneron as similar when they have significantly greater
market capitalizations, revenues, a nd research and development expense than your
company,

 Using the volatility of companies with less than six years of trading data (like Spark
from only February 2015 to May 2015) to estimate expected volatility over the six
year expected term of the options, and

 Tell us how you intend to estimate expected volatility for future stock option grants.

11. Related Party Transactions, page F -40

2. Refer to our prior comment 25.   You disclose on page 166 that certain of your directors
and officers purchased an aggregate of 6.95 million shares of Dimension common stock
for an aggregate price of $695.45.   Please explain why the use of the OPM method based
on a preferred stock issuance was appropriate to value Dimension’s common stock when
there was a contempora neous issuance of its common stock.

3. Please confirm for us whether you received 10,000 shares of Dimension’s common stock
or 10 million shares and how you determined the amount of revenue to recognize.   You
recognized $2.7 million of revenue which would re present 10 million shares at $0.27 per
share.   However, Exhibit 10.18 states that the consideration was 10,000 shares which
would be $2,700 at $0.27 per share.

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 Act of 1933 and
all applicable Securities Act rules require.   Since the company and its management are in
possession of all facts relating to a company’s disclosure, they are resp onsible for the accuracy
and adequacy of the disclosures they have made.

Notwithstanding our comments, in the event you request acceleration of the effective date
of the pending registration statement, please provide a written statement from the company
acknowledging that:

 should the Commission or the staff, acting pursuant to delegated authority, declare the
filing effective, it does not foreclose the Commission from taking any action with respect
to the filing;

 the action of the Commission or the st aff, acting pursuant to delegated authority, in
declaring the filing effective, does not relieve the company from its full responsibility for
the adequacy and accuracy of the disclosure in the filing; and

Kenneth T. Mills
REGENXBIO Inc.
August 26 , 2015
Page 3

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

Please refer to Rules 460 and 461 regarding requests for acceleration.  We will consider a
written request for acceleration of the effective date of the registration statement as confirmation
of the fact that those requesting acceleration are aware of their respective responsibilities under
the Securities Act of 1933 and the Securities Exchange Act of 1934 as they relate to the proposed
public offering of the securities specified in the above registration statement.  Please allow
adequate time for us to review any amendment prior to the requested effective date of the
registration statement.

You may contact Rolf Sundwall at (202) 551 -3105 or Lisa Vanjoske at (202) 551 -3614 if
you have questions regarding comments on the financial statements and related matters.  Please
contact Preston Brewer at (202) 551 -3969, John Krug at (202) 551 -3862 or me a t (202) 551 -3715
with any other questions.

Sincerely,

 /s/ Jeffrey P. Riedler

Jeffrey P. Riedler
Assistant Director
Office of Healthcare and Insurance

cc: Via E -Mail
 Jay K. Hachigian, Esq.
Keith J. Scherer, Esq.
Gunderson Dettmer LLP

July 28, 2015

Via E -Mail
Kenneth T. Mills
Chief Executive Officer
REGENXBIO Inc.
9712 Medical Center Drive, Suite 100
Rockville, MD 20850

Re: REGENXBIO Inc.
Draft Registration Statement on Form S -1
Submitted July 1, 2015
 CIK No. 0001590877

Dear Mr. Mills:

We have reviewed your draft registration statement and have the following comments.  In
some of our comments, we may ask you to provide us with information so we may better
understand your disclosure.

Please respond to this letter by prov iding the requested information and either submitting
an amended draft registration statement or publicly filing your registration statement on
EDGAR.  If you do not believe our comments apply to your facts and circumstances or do not
believe an amendment is appropriate, please tell us why in your response.

After reviewing the information you provide in response to these comments and your
amended draft registration statement or filed registration statement, we may have additional
comments.

Prospectus Summary

1. Please describe the meaning and significance of the following terms at their first use in
this section:
 adeno -associated virus (AAV);
 in vivo ;
 lysosomes;
 orphan drug product designation;
 recessive genetic disease;
 recombinant ; and
 vector.

Kenneth T. Mills
REGENXBIO Inc.
July 28, 2015
Page 2

 2. We note the inclusion of various NAV Technology licensee product candidates in the
table on page 2.  The licensee product candidates are in a preliminary stage of
development and your involvement in such development is limited.  In addition, the
prospectus su mmary provides insufficient context for an adequate understanding of the
license agreements.  Please delete the licensee product candidates table from the
prospectus summary.

3. For your product candidates that are still in ongoing preclinical trials or rese arch, please
revise your pipeline table to shorten the blue bar so that it does not extend to the end of
the stage column.

4. Please revise your disclosure in the prospectus summary to include a general description
of the administration of your AAV gene ther apy in patients. For example, please disclose
how the therapy is delivered, whether the therapy is injected/infused into patients or
whether the patient’s own cells are harvested and isolated, if surgery is required and the
intended injection or surgical s ites in patients.

5. Please revise your disclosure in the Prospectus Summary to provide the number of
individuals who have each of the illnesses for which your product candidates are intended
to treat.

Risk Factors
Because we are developing product candidates for the treatment of diseases, page 13

6. Please define the term “ clinically meaningful”  when the term first appears in the
prospectus.

Our future success depends on our ability to retain key employees…, page 38

7. Please expand your risk factor di sclosure to identify your key executive officers and other
key employees by name and job title.

Product liability lawsuits against us could cause us to incur substantial liabilities…, page 42

8. Please expand your disclosure in this risk factor to quantify the amount of product
liability insurance you carry. Also, for any other type of insurance coverage discussed in
other risk factors, please quantify the amount of insurance you carry.

We will incur increased costs as a result of operating as a public comp any…, page 58

9. Please include an estimate of the additional legal, accounting and other costs you expect
to incur as a public company in your prospectus.  Also provide a separate estimate for
costs associated with the remediation of material weakness identif ied in your internal
control over financial reporting.

Kenneth T. Mills
REGENXBIO Inc.
July 28, 2015
Page 3

 Management’s Discussion and Analysis of Financial Condition and Results of Operations
Research and Development Expense, page 71

10. Please expand your disclosure to include the total costs incurred durin g each period
presented for each project or product candidate separately to provide more transparency
as to the type of expenses incurred.   If you cannot disaggregate the amount of expense by
product candidate, disaggregate the amount by nature of expenses  or in some other
manner.

Critical Accounting Policies and Significant Judgements and Estimates
Stock -Based Compensation
Common Stock Valuation Methodology, page 77

11. We may have additional comments on your accounting for equity issuances including
stock compensation and beneficial conversion features. Once you have an estimated
offering price, please provide us an analysis explaining the reasons for the differences
between recent valuations of your common stock leading up to the IPO and the estimated
offering price.

Broad and Novel Tissue Selectivity, page 102

12. Please revise to briefly disclose why retinal pigment epithelium and the photoreceptor
layer are critical to most inherited retinal degenerations.

Planned Clinical Development of RGX -501, page 107

13. We note your communications with the FDA and NIH.  Please revise your disclosure in
your business section to discuss the dates and substance of any material communications
you have had with each of the FDA and NIH, and any material actions that you have
taken or plan to take in response to the these communications.

License Agreements and Commercial Licenses
The Trustees of the University of Pennsylvania, page 113

14. Please revise your disclosure on page 113 regarding your license agreement with The
Truste es of the University of Pennsylvania to provide the aggregate amounts paid to date
under the agreement and the aggregate future potential milestones to be paid under the
agreement, if any.

GlaxoSmithKline LLC, page 115

15. We note that under the GlaxoSmithK line LLC (GSK) license agreement described on
page 115, exclusivity of such license grant is subject to ``certain rights´´ retained by GSK
and Penn or retained by GSK for the benefit of itself and other third parties. Please
expand your disclosure to descr ibe these rights.

Kenneth T. Mills
REGENXBIO Inc.
July 28, 2015
Page 4

16. Please revise your disclosure on page 115 regarding your license agreement with GSK to
provide the aggregate milestone payments made to date under the agreement.  If no such
payments have been made, please disclose this in the registrati on statement.

Commercial Licenses to NAV Technology Licensees, page 115

17. For each commercial NAV technology license described, please revise to disclose the
total amount of fees you have received to date under each license agreement and
separately state the aggregate milestone payments received to date for each such license
you granted, if applicable.

Other Licenses, page 122

18. Please revise your disclosure on page 122 regarding the license agreements described in
this section to provide:
 the aggregate am ounts paid to date under the agreement with the Regents of the
University of Minnesota; and
 the royalty rates, the aggregate potential milestones to be paid and the aggregate
amounts paid to date under the agreement with ARIAD Pharmaceuticals, Inc.
If no such payments have been made, please disclose this in the registration statement.
Alternatively, explain why you are not required to provide this information.

AAV Vector Expertise, page 123

19. We note that you have two agreements with WuXi AppTec, I nc. regarding the
manufacture of supplies of your product candidates. We also note your disclosure on
page 31 that it would be difficult to find a suitable replacement should your agreement
with WuXi be terminated.  Please expand your disclosure to provide  the material terms of
the agreements, including the parties´ rights and obligations under the agreement, the
duration of the agreement, termination provisions and any payment provisions.

Proprietary Methods, page 124

20. Please revise to clarify the extent  to which the proprietary technology underlying your
NAV technology platform and manufacturing systems were developed in -house as
compared to being licensed from Penn or developed through your SRAs with Penn.

Principal Stockholders, page 166

21. Please update your beneficial ownership table so that it is as of the most recent
practicable date.

Kenneth T. Mills
REGENXBIO Inc.
July 28, 2015
Page 5

 Choice of Forum, page 174

22. We note your disclosure regarding provisions of your amended and restated certificate of
incorporation and amended and restated bylaw s stating that the Delaware Court of
Chancery shall be the sole and exclusive forum for any stockholder bringing specified
actions. Please add a risk factor describing the attendant risks to investors.  For example,
please highlight that such a provision m ay limit a shareholder´s ability to bring a claim in
a judicial forum that it finds favorable for disputes with directors, officers or other
employees, and may discourage lawsuits with respect to such claims.

Statement of Convertible Preferred Stock and P referred Units, and Stockholders’ and Members’
Equity (Deficit), page F -5

23. Tell us how you determined the amount of accretion (decretion) of convertible preferred
stock for the three months ended March 31, 2015 of $(2.28) million and $1.338 million
and the basis for your determination.   Explain why the carrying amount of the preferred is
equal to the redemption amount at March 31, 2015 when it is not redeemable until
December 31, 2019.

Notes to Financial Statements
8. Stock -based Compensation, page F -33

24. Provide us your computation of expected volatility of 64% for stock options granted to
employees.   Include the names of the similar companies you used, the volatilities of each
and why you selected each company as being similar to you.

11. Related Party  Transactions, page F -40

25. Please explain to us your basis for estimating the fair value of shares received from
Dimension Therapeutics, Inc. in the October 2013 license agreement.   Also provide us
your analysis supporting immediate revenue recognition of $ 2.7 million and general and
administrative expense of $2.7 million.   Include references to supporting authoritative
literature.   Similarly, tell us why immediate recognition of two up -front fees in the three
months ended March 31, 2014 is appropriate.

Other Comments

26. Please submit all exhibits as soon as practicable. We may have further comments upon
examination of these exhibits.

27. Please confirm that the graphics included in your registration statement are the only
graphic, visual, or photographic informa tion you will use in your prospectus. If those are
not the only graphics, please provide any additional graphics prior to their use for our
review.

Kenneth T. Mills
REGENXBIO Inc.
July 28, 2015
Page 6

 28. Please supplementally provide us with copies of all written communications, as defined
in Rule 405 under the Securities Act, that you, or anyone authorized to do so on your
behalf, present to potential investors in reliance on Section 5(d) of the Securitie s Act,
whether or not they retain copies of the communications.

29. Your exhibit index indicates that you have submitted a confidential treatment request
with respect to portions of certain of your exhibits. Please note that our comments on
your request for confidential treatment will be provided under separate cover.

You may contact Rolf Sundwall at (202) 551 -3105 or Lisa Vanjoske at (202) 551 -3614 if
you have questions regarding comments on the financial statements and related matters.  Please
contact Pres ton Brewer at (202) 551 -3969, John Krug at (202) 551 -3862 or me at (202) 551 -3715
with any other questions.

Sincerely,

 /s/ Jeffrey P. Riedler

Jeffrey P. Riedler
Assistant Director

cc: Via E -Mail
 Jay K. Hachigian, Esq.
Keith J. Scherer, Esq.
Gunderson Dettmer LLP