SecProbe.io

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 CytomX Therapeutics, Inc.
 151 Oyster Point Blvd., Suite 400
 South San Francisco, CA 94080
 August 14, 2025 VIA EDGAR
 Securities and Exchange Commission Division of Corporation
Finance 100 F Street, N.E. Washington, DC 20549
 Attention:  Chris Edwards

 Re:
 CytomX Therapeutics, Inc.
 Registration Statement on Form S-3 (File
 No. 333-289365) To the addressee set forth above:
 In accordance with Rule 461 of Regulation C of the General Rules and Regulations under the Securities Act of 1933, as amended, we hereby
request acceleration by the Securities and Exchange Commission of the effective date of the above-referenced Registration Statement on Form S-3 (the “Registration Statement”) of CytomX Therapeutics,
Inc. We respectfully request that the Registration Statement become effective as of 4:00 p.m., Washington, D.C. time, on August 18, 2025, or as soon as practicable thereafter.
 Thank you for your assistance in this matter.

 Very truly yours,

 CytomX Therapeutics, Inc.

 By:

 /s/ Christopher W. Ogden

 Christopher W. Ogden

 Chief Financial Officer

 CC:
 Sean A. McCarthy, CytomX Therapeutics, Inc.
 Mark V. Roeder, Latham & Watkins LLP
 John C. Williams, Latham & Watkins LLP

<DOCUMENT>
<TYPE>TEXT-EXTRACT
<SEQUENCE>2
<FILENAME>filename2.txt
<TEXT>
 August 13, 2025

Sean A. McCarthy, D.Phil.
President and Chief Executive Officer
CytomX Therapeutics, Inc.
151 Oyster Point Blvd., Suite 400
South San Francisco, CA 94080

 Re: CytomX Therapeutics, Inc.
 Registration Statement on Form S-3
 Filed August 7, 2025
 File No. 333-289365
Dear Sean A. McCarthy D.Phil.:

 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 Chris Edwards at 202-551-6761 with any questions.

 Sincerely,

 Division of
Corporation Finance
 Office of Life
Sciences
cc: Mark V. Roeder, Esq.
</TEXT>
</DOCUMENT>

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 CytomX Therapeutics, Inc.

151 Oyster Point Blvd., Suite 400

South San Francisco, CA 94080

 August 15,
2024

 VIA EDGAR

 Securities and Exchange
Commission

 Division of Corporation Finance

 100 F Street,
N.E.

 Washington, DC 20549

 Attention:  Tamika
Sheppard

Re:
 CytomX Therapeutics, Inc.

Registration Statement on Form S-3 (Registration
No. 333-281433)

 Ladies and Gentlemen:

In accordance with Rule 461 of Regulation C of the General Rules and Regulations under the Securities Act of 1933, as amended, we hereby
request acceleration by the Securities and Exchange Commission of the effective date of the above-referenced Registration Statement on Form S-3 (the “Registration Statement”) of CytomX Therapeutics,
Inc. We respectfully request that the Registration Statement become effective as of 4:00 p.m., Washington, D.C. time, on August 19, 2024, or as soon as practicable thereafter.

Thank you for your assistance in this matter.

Very truly yours,

CytomX Therapeutics, Inc.

By:

 /s/ Lloyd A. Rowland

Lloyd A. Rowland

Senior Vice President and General Counsel

CC:
 Sean A. McCarthy, CytomX Therapeutics, Inc.

Chris Ogden, CytomX Therapeutics, Inc.

Mark V. Roeder, Latham & Watkins LLP

[Signature Page to CytomX Therapeutics, Inc. Acceleration Letter]

August 15, 2024
Chris Ogden
Chief Financial Officer
CytomX Therapeutics, Inc.
151 Oyster Point Blvd., Suite 400
South San Francisco, CA 94080
Re:CytomX Therapeutics, Inc.
Registration Statement on Form S-3
Filed August 9, 2024
File No. 333-281433
Dear Chris Ogden:
            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 Tamika Sheppard at 202-551-8346 with any questions.
Sincerely,
Division of Corporation Finance
Office of Life Sciences
cc:Mark Roeder

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 CytomX Therapeutics, Inc.

151 Oyster Point Blvd., Suite 400

South San Francisco, California 94080

August 23, 2023

 Via EDGAR

United States Securities and Exchange Commission

 Division of
Corporation Finance

 Office of Life Sciences

 100 F Street,
N.E.

 Washington, D.C. 20549

 Attention: Jimmy McNamara

Re:
 CytomX Therapeutics, Inc.

 Registration Statement on Form S-3

 Filed August 16, 2023

 Registration No. 333-274010

Dear Mr. McNamara:

 Pursuant to Rule 461(a)
under the Securities Act of 1933, as amended, CytomX Therapeutics, Inc. (the “Company”) hereby respectfully requests that the effective date of the above-referenced Registration Statement be accelerated by the Securities and
Exchange Commission to 4:00 p.m. Washington D.C. time on August 25, 2023, or as soon as practicable thereafter, or at such later time as the Company or its counsel may request via telephone call to the staff. Please contact Mark V. Roeder of
Latham & Watkins LLP, counsel to the Company, at (650) 463-3043, or in his absence, John C. Williams of Latham & Watkins LLP at (415) 395-8223, to
provide notice of effectiveness, or if you have any other questions or concerns regarding this matter.

 Thank you for your assistance in
this matter.

*        *        *
 *

 Sincerely,

CytomX Therapeutics, Inc.

 By:

 /s/ Lloyd A. Rowland

 Lloyd A. Rowland

 General Counsel

cc:
 Sean A. McCarthy, CytomX Therapeutics, Inc.

 Christopher W. Ogden, CytomX Therapeutics, Inc.

 Mark V. Roeder, Latham & Watkins LLP

 John C. Williams, Latham & Watkins LLP

United States securities and exchange commission logo
August 22, 2023
Sean McCarthy, D.Phil.
Chief Executive Officer
CytomX Therapeutics, Inc.
151 Oyster Point Blvd., Suite 400
South San Francisco, CA 94080
Re:CytomX Therapeutics, Inc.
Registration Statement on Form S-3
Filed August 16, 2023
File No. 333-274010
Dear Sean McCarthy:
            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 Dillon Hagius at 202-551-7967 with any questions.
Sincerely,
Division of Corporation Finance
Office of Life Sciences
cc:       Mark Roeder

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 CytomX Therapeutics, Inc.

151 Oyster Point Blvd., Suite 400

South San Francisco, CA 94080

August 16, 2021

 VIA EDGAR

Securities and Exchange Commission

 Division of Corporation
Finance

 100 F Street, N.E.

 Washington, DC 20549

 Attention:

Deanna Virginio

           Re:

 CytomX Therapeutics, Inc.

Registration Statement on Form S-3 (Registration No. 333-258510)

 Ladies and Gentlemen:

In accordance with Rule 461 of Regulation C of the General Rules and Regulations under the Securities Act of 1933, as amended, we hereby
request acceleration by the Securities and Exchange Commission of the effective date of the above-referenced Registration Statement on Form S-3 (the “Registration Statement”) of CytomX Therapeutics,
Inc. We respectfully request that the Registration Statement become effective as of 4:00 p.m., Washington, D.C. time, on August 18, 2021, or as soon as practicable thereafter.

Thank you for your assistance in this matter.

Very truly yours,

 CytomX Therapeutics, Inc.

By:

/s/ Lloyd A. Rowland

Lloyd A. Rowland

Senior Vice President and General Counsel

 CC:

Sean A. McCarthy, CytomX Therapeutics, Inc.

 Carlos Campoy, CytomX Therapeutics, Inc.

Mark V. Roeder, Latham & Watkins LLP

United States securities and exchange commission logo
August 12, 2021
Sean A. McCarthy, D.Phil.
President and Chief Executive Officer
CytomX Therapeutics, Inc.
151 Oyster Point Blvd., Suite 400
South San Francisco, CA 94080
Re:CytomX Therapeutics, Inc.
Registration Statement on Form S-3
Filed August 5, 2021
File No. 333-258510
Dear Dr. McCarthy:
            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 Deanna Virginio at 202-551-4530 with any questions.
Sincerely,
Division of Corporation Finance
Office of Life Sciences
cc:       Mark V. Roeder, Esq.

United States securities and exchange commission logo
April 29, 2020
Lloyd Rowland, Esq.
General Counsel
CytomX Therapeutics, Inc.
151 Oyster Point Boulevard, Suite 400
South San Francisco, CA 94080
Re:CytomX Therapeutics, Inc.
Preliminary Proxy Materials on Schedule 14A
Filed on April 17, 2020
File No. 001-37587
Dear Mr. Rowland:
            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
cc:       Mark Roeder, Esq.

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505 Montgomery Street, Suite 2000
 San Francisco, California 94111-6538

Tel: +1.415.391.0600 Fax: +1.415.395.8095

www.lw.com

FIRM / AFFILIATE OFFICES

 April 28, 2020

Beijing
 Boston

 Brussels

Century City

 Chicago

Dubai

 Düsseldorf

Frankfurt

 Hamburg

Hong Kong

 Houston

London

 Los Angeles

Madrid

 Milan

Moscow
 Munich

 New York

Orange County

 Paris

Riyadh

 San Diego

San Francisco

 Seoul

Shanghai

 Silicon Valley

Singapore

 Tokyo

Washington, D.C.

 Via EDGAR

Paul Fischer

 United States Securities and Exchange Commission

 Division of Corporation Finance

 100 F Street, N.E.

Washington, D.C. 20549

Re:
 CytomX Therapeutics, Inc.

 Preliminary Proxy Statement on Schedule 14A filed April 17, 2020

 File No. 001-37587

Dear Mr. Fischer:

 On behalf of CytomX
Therapeutics, Inc. (the “Company”), we are responding to the oral comment of the staff (the “Staff”) of the U.S. Securities and Exchange Commission (the “Commission”) conveyed to us on
April 27, 2020 regarding the review of the above-referenced filing of the Company. We have set forth below our response to the inquiry raised by the Staff. For ease of reference, we have included the Staff’s comment in its entirety in bold
and italicized text preceding our response.

 April 28, 2020

 Page
 2

 Preliminary Proxy Statement on Schedule 14A

1.
 We note that under Proposal 3 you’re seeking to increase the number of authorized common shares from
75 million to 150 million shares. Please revise to clarify, if true, that the Board currently has no specific designated purpose for the increased number of common shares. To the extent the Board does have a specific designated purpose
intended for the proposed increase to the common shares, please revise to so disclose and include all applicable disclosures required by Note A to Schedule 14A.

Response: The Company respectfully acknowledges the Staff’s comment and advises the Staff that the Company’s board of
directors currently has no specific designated purpose for the increase in authorized shares of common stock. In response to the Staff’s comment, the Company confirms that it will revise page 13 of the Company’s Preliminary Proxy Statement
by adding disclosure to its Definitive Proxy Statement to be filed on April 29, 2020 under the “Proposal 3 Approval to Amend our Amended and Restated Certification of Incorporation to Increase the Number of Shares of Authorized Common
Stock From 75,000,000 to 150,000,000—Board Recommendation” section as follows (emphasis added):

 “The Board believes that
additional authorized shares of common stock would give the Company the necessary flexibility to issue shares for various corporate purposes, including, in particular, capital-raising or financing transactions, and enable the Company to take timely
advantage of market conditions and opportunities. Other corporate purposes for which the additional authorized shares could be used include, but are not limited to, potential strategic transactions, including mergers, acquisitions, and other
business combinations; future development of the Company’s pipeline of products, future grants and awards under equity compensation plans; stock splits and stock dividends; and other general corporate working capital needs. The Board
currently has no specific designated purpose for the increase in authorized shares of common stock. As a general matter, the Board would be able to issue the additional authorized shares of common stock at its discretion from time to time,
subject to and as limited by, rules or listing requirements of the Nasdaq or any other then applicable securities exchange, and without further action or approval of the Company’s stockholders. The discretion of the Board, however, would be
subject to any other applicable rules and regulations in the case of any particular issuance or reservation for issuance that might require our stockholders to approve such transaction.”

In addition, the Company hereby acknowledges that (i) it is responsible for the adequacy and accuracy of the disclosure in the filing,
(ii) the Staff’s 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 (iii) 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 further
questions or comments, please do not hesitate to contact me by telephone at (650) 463-3043 or by fax at (650) 463-2600 with any questions or comments regarding this
correspondence.

Sincerely,

/s/ Mark V. Roeder

 Mark V. Roeder

 of LATHAM & WATKINS
LLP

cc:
 Sean McCarthy, CytomX Therapeutics, Inc.

 Lloyd Rowland, CytomX Therapeutics, Inc.

 Miles Jennings, Latham & Watkins LLP

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 CytomX Therapeutics, Inc.

151 Oyster Point Blvd., Suite 400

South San Francisco, CA 94080

February 8, 2019

 VIA EDGAR

Securities and Exchange Commission

 Division of Corporation
Finance

 100 F Street, N.E.

 Washington, DC 20549

Attention:
 Joseph McCann, Staff Attorney

Re:
 CytomX Therapeutics, Inc.

 Registration Statement on Form S-3 (Registration
No. 333-228203)

 Ladies and Gentlemen:

In accordance with Rule 461 of Regulation C of the General Rules and Regulations under the Securities Act of 1933, as amended, we hereby
request acceleration by the Securities and Exchange Commission of the effective date of the above-referenced Registration Statement on Form S-3 (the “Registration Statement”) of CytomX Therapeutics,
Inc. We respectfully request that the Registration Statement become effective as of 4:00 p.m., Washington, D.C. time, on February 11, 2019, or as soon as practicable thereafter.

Thank you for your assistance in this matter.

Very truly yours,

CytomX Therapeutics, Inc.

By:

 /s/ Lloyd A. Rowland

Lloyd A. Rowland

General Counsel

CC:
 Sean A. McCarthy, CytomX Therapeutics, Inc.

 Debanjan Ray, CytomX Therapeutics, Inc.

 Mark V. Roeder, Latham & Watkins LLP

CORRESP
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 CytomX Therapeutics, Inc.

151 Oyster Point Blvd., Suite 400

South San Francisco, CA 94080

March 17, 2017

 VIA EDGAR

Securities and Exchange Commission

 Division of Corporation
Finance

 100 F Street, N.E.

 Washington, DC 20549

Attention:
Suzanne Hayes, Assistant Director, Office of Healthcare and Insurance

Mary Beth Breslin, Senior Attorney

Johnny Gharib, Attorney

Re:
CytomX Therapeutics, Inc.

Registration Statement on Form S-3 (Registration No. 333-216567)

 Ladies and Gentlemen:

In accordance with Rule 461 of Regulation C of the General Rules and Regulations under the Securities Act of 1933, as amended, we hereby
request acceleration by the Securities and Exchange Commission (the “Commission”) of the effective date of the above-referenced Registration Statement on Form S-3 (the “Registration Statement”) of CytomX Therapeutics, Inc. (the
“Company”). We respectfully request that the Registration Statement become effective as of 5:00 p.m., Washington, D.C. time, on March 20, 2017, or as soon as practicable thereafter.

The Company acknowledges the following:

•

should the Commission or the staff of the Commission (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 Staff, 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

•

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.

 Thank you for your assistance in this matter.

Very truly yours,

CytomX Therapeutics, Inc.

By:

/s/ Robert C. Goeltz II

Robert C. Goeltz II

Chief Financial Officer and Secretary

[Signature Page to CytomX Therapeutics, Inc. Acceleration Letter]

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 CytomX Therapeutics, Inc.

151 Oyster Point Blvd., Suite 400

South San Francisco, CA 94080

December 5, 2016

 VIA EDGAR

Securities and Exchange Commission

 Division of Corporation
Finance

 100 F Street, N.E.

 Washington, DC 20549

Attention:
Suzanne Hayes, Assistant Director, Office of Healthcare and Insurance

Mary Beth Breslin, Senior Attorney

Johnny Gharib, Attorney

Re:
CytomX Therapeutics, Inc.

Registration Statement on Form S-3 (Registration No. 333-214418)

 Ladies and Gentlemen:

In accordance with Rule 461 of Regulation C of the General Rules and Regulations under the Securities Act of 1933, as amended, we hereby
request acceleration by the Securities and Exchange Commission (the “Commission”) of the effective date of the above-referenced Registration Statement on Form S-3 (the “Registration Statement”) of CytomX Therapeutics, Inc. (the
“Company”). We respectfully request that the Registration Statement become effective as of 4:00 p.m., Washington, D.C. time, on December 7, 2016, or as soon as practicable thereafter.

The Company acknowledges the following:

•

should the Commission or the staff of the Commission (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 Staff, 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

•

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.

 Thank you for your assistance in this matter.

Very truly yours,

CytomX Therapeutics, Inc.

By:

/s/ Robert C. Goeltz II

 Robert C. Goeltz II

 Chief Financial Officer
and Secretary

 [Signature Page to
CytomX Therapeutics, Inc. Acceleration Letter]

Mail Stop 4546

November 21, 2016

Sean A. McCarthy
President and Chief Executive Officer
CytomX Therapeutics, Inc.
151 Oyster Point Blvd., Suite 400
South San Francisco, California 94080

Re: CytomX Therapeutics, Inc.
  Registration Statement on Form S-3
Filed November 3, 2016
  File No. 333 -214418

Dear Mr. McCarthy:

We have limited our review of your registration statement to the issue we have addressed
in our comment.  In our comment, 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 comment a pplies to your facts and
circumstances or do not believe an amendment is appropriate, please tell us why in your
response.

After reviewing any amendment to your registration statement and the information you
provide in response to our comment, we may ha ve additional comments.

Sales Agreement Prospectus Cover Page

1. Your disclosure on the sales agreement prospectus cover page states that sales may be
made “to or through a market maker other than on an exchange or otherwise, in
negotiated transactions a t market prices prevailing at the time of sale or at prices related
to such prevailing market prices, and/or any other method permitted by law, including in
privately negotiated transactions.”  Please tell us whether sales made to or through a
market maker , sales made in negotiated transactions at market prices or at prices related
to prevailing market prices or sales made in privately -negotiated transaction satisfy the
“at the market offering” definition under Rule 415.  If any sales method does not
consti tute a sales method that is deemed to be an “at the market offering” as defined in
Rule 415 or if any material information with respect to a particular offering has been
omitted, please confirm that you will file an additional prospectus supplement at the time
of such sales or tell us why such additional filing would not be necessary.

Sean A. McCarthy
CytomX Therapeutics, Inc.
November 21 , 2016
Page 2

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.

Refer to Rules 460 and 461 regarding requests for acceleration.  Please allow adequate
time for us to review any amendment prior to the requested effective date of the registration
statement.

You may contact Johnny Gharib a t (202) 551 -3170 or Mary Beth Breslin at (202) 551 -
3625 with any questions.

Sincerely,

 /s/ Mary Beth Breslin for

 Suzanne Hayes
Assistant Director
Office of Healthcare and Insurance

cc: Mark V. Roeder, Esq.
 Latham & Watkins LLP

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Correspondence

 SIDLEY AUSTIN LLP

 1001 PAGE MILL ROAD

BUILDING 1

 PALO ALTO, CA 94304

(650) 565 7000

 (650) 565 7100 FAX

 BEIJING

 BOSTON

BRUSSELS

 CENTURY CITY

CHICAGO

 DALLAS

GENEVA

 FOUNDED 1866

 HONG KONG

 HOUSTON

LONDON

 LOS ANGELES

NEW YORK

 PALO ALTO

SAN FRANCISCO

 SHANGHAI

 SINGAPORE

SYDNEY

 TOKYO

WASHINGTON, D.C

 October 5, 2015

 Via EDGAR and Courier

 Securities and Exchange
Commission

 Division of Corporation Finance

 100 F Street,
N.E.

 Washington, DC 20549

 Attn: Ruairi Regan/Pamela Howell

Re:
Acceleration Request for CytomX Therapeutics, Inc.

Registration Statement on Form S-1 (File No. 333-206658)

 Ladies and Gentlemen:

Pursuant to Rule 461 under the Securities Act of 1933, as amended, we attach the requests of our client, CytomX Therapeutics, Inc., and of the
underwriters that the effectiveness of the above-referenced Registration Statement be accelerated to 4:00 P.M., Washington, D.C. time, on October 7, 2015, or as soon as practicable thereafter. We ask, however, that the Securities and
Exchange Commission staff not accelerate such effectiveness until we speak with you on that date.

 Please direct any questions to me
by telephone ((650) 565-7111) or by e-mail (szucker@sidley.com) or my colleague, Robert A. Ryan, by telephone ((212) 839-5931) or by e-mail (rryan@sidley.com).

Sincerely,

/s/ Sam Zucker

Sam Zucker, Esq.

(Attachments)

 Sidley Austin (CA) LLP is
a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in affiliation with other Sidley Austin partnerships

 [CytomX Therapeutics, Inc. Letterhead]

October 5, 2015

 Via
EDGAR and Courier

 Securities and Exchange Commission

Division of Corporation Finance

 100 F Street, N.E.

Washington, DC 20549

 Attn: Ruairi Regan/Pamela Howell

Re:
Acceleration Request for CytomX Therapeutics, Inc.

Registration Statement on Form S-1 (File No. 333-206658)

 Ladies and Gentlemen:

Pursuant to Rule 461 under the Securities Act of 1933, as amended, CytomX Therapeutics, Inc. (the “Company”), hereby requests that
the effective date of the Company’s Registration Statement on Form S-1, Registration Number 333-206658 (the “Registration Statement”) be accelerated so that the Company’s Registration Statement will become effective at 4:00 P.M.,
Washington, D.C. time, on October 7, 2015, or as soon thereafter as practicable. In this regard, the Company is aware of its obligations under the Securities Act as they relate to the above-referenced Registration Statement and acknowledges:

•

should the Securities and Exchange Commission (the “SEC”) or its staff (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;

•

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

•

the Company may not assert Staff comments and the declaration of the Registration Statement’s effectiveness as a defense in any proceeding initiated by the Commission or any person under the federal securities laws
of the United States.

 It would be appreciated if, as soon as the Registration Statement is declared effective, you would so
inform Sam Zucker of Sidley Austin LLP at (650) 565-7111.

 Very truly yours,

/s/ Cynthia J. Ladd

Cynthia J. Ladd, Esq.

 MERRILL LYNCH, PIERCE, FENNER & SMITH

     INCORPORATED

 One Bryant Park

New York, New York 10036

 JEFFERIES LLC

520 Madison Avenue

 New York, New York 10022

COWEN AND COMPANY, LLC

 599 Lexington Avenue, 27th Floor

 New York, New York 10022

VIA EDGAR

October 5, 2015

 U.S. Securities and Exchange Commission

Division of Corporation Finance

 100 F. Street, N.E. Washington,
D.C. 20549-3628

Attention:
John Reynolds

Ruairi Regan

Pamela Howell

Re:
CytomX Therapeutics, Inc. Registration Statement on Form S-1

File No. 333-206658

Acceleration Request

 Ladies and Gentlemen:

As representatives of the several underwriters of the proposed public offering of 6,666,667 shares of common stock of CytomX Therapeutics, Inc.
(the “Company”), we hereby join the Company’s request to accelerate the date of the above-referenced Registration Statement, as then amended, effective as of 4:00 p.m. (New York time) on October 7, 2015, or as soon thereafter as
is practicable.

 Pursuant to Rule 460 under the Securities Act of 1933, as amended, we wish to advise you that we have distributed
approximately 1,709 copies of the Company’s preliminary prospectus, dated September 28, 2015, through the date hereof, to underwriters, dealers, institutions and others. The undersigned have been informed by the participating underwriters
that they have complied and will continue to comply with Rule 15c2-8 under the Securities Exchange Act of 1934, as amended.

 (Signature
Page Follows)

 Very truly yours,

 MERRILL LYNCH, PIERCE, FENNER & SMITH

   INCORPORATED

 JEFFERIES LLC

 COWEN AND
COMPANY, LLC

 As representatives of the several underwriters

By:

 MERRILL LYNCH, PIERCE, FENNER & SMITH

   INCORPORATED

By:

/s/ Bradley Wolff

Name: Bradley Wolff

Title:   Managing Director

By:

JEFFERIES LLC

By:

/s/ Ashley Delp Walker

Name: Ashley Delp Walker

Title:   Managing Director

By:

COWEN AND COMPANY, LLC

By:

/s/ Jason Fenton

Name: Jason Fenton

Title:   Managing Director

CORRESP
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Corresp

 Sidley Austin LLP

 1001 Page Mill Road

Building 1

 Palo Alto, CA 94304

(650) 565 7000

 (650) 565 7100 FAX

 BEIJING

 BOSTON

BRUSSELS

 CENTURY CITY

CHICAGO

 DALLAS

GENEVA

FOUNDED 1866

 HONG KONG

 HOUSTON

LONDON

 LOS ANGELES

NEW YORK

 PALO ALTO

SAN FRANCISCO

 SHANGHAI

 SINGAPORE

SYDNEY

 TOKYO

WASHINGTON, D.C.

 September 21, 2015

Via EDGAR and Courier

 Securities and Exchange Commission

 Division of Corporation Finance

 100 F Street, N.E.

Washington, DC 20549

 Attn: Ruairi Regan/Pamela Howell

Re:
CytomX Therapeutics, Inc.

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

Filed September 11, 2015

Response dated September 18, 2015

File No. 333-206658

 Ladies and Gentlemen:

On behalf of CytomX Therapeutics, Inc. (the “Company”), we hereby respond (the “Response Letter”) to the
supplemental comments raised by the staff (the “Staff”) of the Division of Corporation Finance of the Securities and Exchange Commission (the “Commission”) in its letter dated September 18, 2015 relating to the
draft marked pages of the Company’s Amendment No. 3 (“Amendment No. 3”) to the registration statement on Form S-1 (the “Registration Statement”), which Registration Statement was filed under the
Securities Act of 1933, as amended (the “Securities Act”), on August 28, 2015 and amended by Amendment No. 1 to the Registration Statement on September 11, 2015. For your convenience, the Staff’s comments are
included in this letter and are followed by the applicable responses. We are also providing draft marked pages to Amendment No. 3 in connection with this Response Letter that reflect (i) further revisions based on the responses set forth herein
and (ii) certain other minor revisions to Amendment No. 3.

 The Offering, page 8

1.
You disclose that the number of shares to be outstanding after the offering is based on 28,259,170 shares of common stock outstanding as of August 31, 2015 which includes the conversion of all shares of
preferred stock and the net exercise of all outstanding warrants to purchase shares of preferred stock. Please tell us how you determined the amount of common shares to be outstanding in view of the 27,135,453 converted preferred shares and 65,884
purchase warrants reflected in footnote 1 to the table on page 11.

 Sidley Austin (CA)
LLP is a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in affiliation with other Sidley Austin partnerships.

 September 21, 2015

  Page
 2

 Response: The Company respectfully advises the Staff that the 28,259,170 shares
presented on page 8 of Amendment No. 3 is calculated as the sum of 27,135,453 shares representing the preferred stock converted to shares of common stock in connection with the closing of the transaction, 65,884 shares representing the shares
issuable upon net exercise of the warrants in connection with the closing of the transaction and 1,057,833 shares of common stock that were outstanding as of August 31, 2015.

Dilution, page 58

2.
You state that your historical net tangible book value per share is comprised of your total tangible assets less total liabilities, less preferred stock, divided by the number of your outstanding shares of common
stock. Please specify the assets that you are excluding from your total tangible assets and provide your calculations that support your historical, pro forma and pro forma as adjusted net tangible book value per share as of June 30, 2015.

 Response: The Company has updated the disclosure in Amendment No. 3 regarding dilution to specify the
exclusion of intangible assets and goodwill in its calculation of net tangible book value, and has revised its calculations accordingly. These calculations are attached hereto as Appendix A. Please see pages 58-59 of Amendment No. 3.

 Financial Statements

 14. Stock Option
Plans, page F-29

3.
Your disclosures indicate that you granted 697,557 stock options in 2014 with a weighted average exercise price of $1.449 and an additional 1,290,705 stock options with a weighted average exercise price of $2.621
during the six months ended June 30, 2015. Please address the following points:

•

Provide us with further details for stock and options issued on dates within the last year including number of shares/options granted, grant dates, and exercise prices.

•

Tell us the methods used and the material assumptions involved in determining the fair value of the company’s shares in relation to the stock or options issued leading up to the offering.

•

To the extent estimated IPO price differs significantly from the most recent fair value determined for your common shares, please tell us the significant factors and events contributing to the changes in fair value
of the underlying stock leading up to the expected IPO.

 September 21, 2015

  Page
 3

 Response:

The following table summarizes by grant date the number of shares of the Company’s common stock (“Common Stock”) underlying stock options
granted by the Company during the twelve months preceding, and the period subsequent to, the most recent balance sheet date included in the Registration Statement, as well as the associated per Common Share exercise price and the estimated fair
value per share of the Common Stock used to determine stock-based compensation expense for financial reporting purposes.

 Grant date

Number of shares
underlying
Common Stock
options granted

Exercise price per
share of Common
Stock

Estimated fair value
per share of
Common Stock

 October 9, 2014

249,549

$
1.51

$
1.51

 February 9, 2015

736,026

$
1.57

$
2.77
(1)

 April 1, 2015

89,878

$
1.57

$
4.22
(1)

 May 7, 2015

464,801

$
4.47

$
5.35
(1)

 July 21, 2015

539,041

$
6.61

$
9.61
(2)

 August 7, 2015

9,237

$
6.61

$
15.0
(2)

 August 24, 2015

5,821

$
6.61

$
15.0
(2)

 August 26, 2015

986,172

$
6.61

$
15.0
(2)

 August 28, 2015

435,407

$
6.61

$
15.0
(2)

(1)
At the time of the option grants on February 9, 2015, April 1, 2015 and May 7, 2015, the Company’s board of directors determined the fair value of the Common Stock after taking into account the
Company’s most recently available contemporaneous valuations of its Common Stock as of such grant date, as well as additional factors that may have changed since the date of such contemporaneous valuation through the date of grant. However, as
described below, for financial reporting purposes the fair value of Common Stock at the date of these grants were adjusted in connection with a retrospective fair value assessment.

(2)
 At the time of July 21, 2015, August 7, 2015, August 24, 2015, August 26, 2015 and August 28, 2015 stock
option grants, the Company initially determined the estimated fair value of its Common Stock in connection with the grants based on its most recent available valuation as of June 30, 2015 . On August 28, 2015, subsequent to the grant
dates, the underwriters selected by

 September 21, 2015

  Page
 4

the Company to conduct its initial public offering (“IPO”) presented a preliminary analysis of the proposed valuation of the Company. Based on the underwriters’ presentation
and input from the Company’s management and board of directors, a preliminary price range of $15.00 to $17.00 per share of Common Stock (the “Preliminary IPO Price Range”) was derived. In light of the determination of the
Preliminary IPO Price Range, for financial reporting purposes the Company retrospectively evaluated the fair value of its Common Stock with respect to the stock options granted in July and August 2015. In connection with this reassessment and the
proximity to the proposed IPO, the Company has determined to utilize the lower end of the Preliminary IPO Price Range, $15.00 per share, as the fair value of its Common Stock for purposes of recording stock-based compensation expense for the grants
made in August 2015. With respect to the July 21, 2015 grants, the Company considered the increase in the fair value of Common Stock between June 30, 2015 to August 28, 2015 and determined the July 21, 2015 value to be $9.61
using the ratable approach, as no single event was responsible for the increase in the fair value of the Company’s Common Stock between June 30, 2015 and August 28, 2015.

Historical Determination of Fair Value of Common Stock on Grant Dates

As previously disclosed, the Company determined the fair value of its 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 “AICPA Practice Guide”). In addition, the
Company’s board of directors also considered various objective and subjective factors, along with input from management and independent third-party valuations, to determine the fair value of its Common Stock as disclosed in the Registration
Statement.

 As described in more detail in the Registration Statement, the Company utilized the probability-weighted expected return
method (“PWERM”), and option pricing method (“OPM”), which are accepted valuation methods under the AICPA Practice Guide, for determining the fair value of its Common Stock. The PWERM method is appropriate for a
company expecting a liquidity event, but where, due to market or other factors, the likelihood of completing the liquidity event is uncertain. The PWERM involves a forward-looking analysis of the possible future outcomes of the enterprise. The
future outcomes considered under the PWERM included non-IPO market based outcomes as well as IPO scenarios. The fair value of the enterprise determined using the IPO and non-IPO scenarios would be weighted according to the Company’s board of
directors’ estimate of the probability of each scenario.

 Over time, a combination of factors caused changes in the fair value of the
Company’s Common Stock. The following summarizes the changes in value during the twelve months preceding, and the period subsequent to, the most recent balance sheet date included in the Registration Statement.

 September 21, 2015

  Page
 5

 October 9, 2014. During the period leading to the October 9, 2014
grants, the Company continued to make progress in its pre-clinical product portfolio. In April 2014, the Company closed an additional tranche of Series B-1 preferred stock financing. In May 2014, the Company entered into a worldwide collaboration
agreement with Bristol-Myers Squibb (“BMS”) and granted BMS exclusive worldwide rights to develop and commercialize Probodies for up to four oncology targets.

The Company estimated the fair value of Common Stock based on the valuation of its Common Stock that indicated that the fair value of the
Common Stock on that date was $1.51 per share. This contemporaneous valuation used the PWERM methodology to develop a probability weighted estimate of the value of the Company’s Common Stock based on the assessment of several possible future
scenarios that were weighted at 10% for the IPO liquidity event, 50% for an M&A scenario and 40% potential dissolution scenario. Prior to the collaboration, the Company had less than one year of funding for operations. Based on the
Company’s two major collaborations with Pfizer and BMS, as well as M&A trends in the biotechnology industry, the Company anticipated a lower probability of an exit through IPO compared to the M&A scenario.

In light of the valuation and factors existing as of the grant date, the Company’s board of directors determined that the estimated fair
value of the Company’s Common Stock of $1.51 per share to be reasonable.

 The Company also notes that it issued shares of its Series
C preferred stock financing at $5.31 per share on December 21, 2014. The Series C preferred stock has certain additional rights, preferences and privileges relative the Company’s Common Stock, including a senior liquidation preference,
preferential dividend rights, protective voting rights and conversion rights. Nonetheless, for sensitivity purposes, the Company submits that if the Series C preferred stock price was retrospectively used to estimate the fair value of the Common
Stock underlying the October 9, 2014 grants, the incremental stock based compensation would be immaterial, amounting to less than $100,000 for the year ended December 31, 2014 with similar impact during the six months ended June 30,
2015.

 Reassessment of Fair Value in Connection With Preparation of Interim Financial Statements

On June 28, 2015, subsequent to the closing of its Series D preferred stock financing, the Company held an organizational meeting relating
to a proposed IPO. In connection with preparing for a proposed IPO and the preparation of the Company’s interim consolidated financial statements to be included in the Registration Statement, the Company reexamined the estimated fair value of
its Common Stock associated with stock option issuances at February 9, 2015, April 1, 2015 and May 7, 2015, using revised probability weighting of liquidity events in light of the Company’s stage of development and its
prospects for an IPO.

 September 21, 2015

  Page
 6

 February 9, 2015. For purposes of the February 9, 2015 stock option
grant issuance, the Company initially determined the estimated fair value of its Common Stock to be $1.57 per share, based on its December 31, 2014 valuation, the most recent available valuation. As a result of the reassessment, using a ratable
approach for the increase in the fair value of Common Stock from December 31, 2014 through March 31, 2015 (discussed in further detail below), the Company determined the February 9, 2015 fair value at $2.77 per share for financial
reporting purposes.

 April 1, 2015. The Company performed valuation of its Common Stock as of March 31,
2015 that indicated that the fair value of the Common Stock on that date was $4.22 per share. In this valuation, the Company used the PWERM methodology to develop a probability weighted estimate of the value of the Company’s Common Stock based
on the assessment of several possible future scenarios, in which an IPO liquidity event was weighted at 25%, an M&A scenario was weighted at 50% and a potential dissolution scenario was weighted at 25%. The Company determined that an exit
through an IPO became more likely relative to prior periods and that the exit values would be higher based on the fact that the market continued to be receptive to healthcare initial public offerings, with 13 transactions pricing in the first
quarter of 2015 and the Nasdaq Biotech Index increasing during the same period. The Company’s board of directors considered several additional factors in its determination of the fair value of the Company’s Common Stock, including the
following:

•

In January 2015, the Company hired Dr. Michael Kavanaugh as its Chief Scientific Officer. Prior to joining the Company, Dr. Kavanaugh led the growth of the therapeutic pipelines at Five Prime Therapeutics,
Novartis Institutes of Biomedical Research and Chiron Corporation. Hiring Dr. Kavanaugh was the key to the Company’s plans for successful product development and expanding partnerships with pharmaceutical companies.

•

In January and February 2015, the Company generated preclinical proof of concept for several of its targets. For PD-L1 target, Company generated proof of concept data for both tumor efficacy and safety models. For
targets CD166, CD71 and ITGA3, the Company generated proof of concept data for the tumor efficacy models.

•

In March 2015, the Company received confirmation of intact Probody expression in the Chinese Hamster Ovary or CHO system. This system is a widely-used standard for manufacturing antibodies. This was an important
progress to ensure feasibility and efficiency of the manufacturing process.

 In light of these developments, the Company’s board of
directors determined that the estimated fair value of the Company’s Common Stock ha

Mail Stop 3561

        September 18, 2015

Sean A. McCarthy, D.Phil.
President and Chief Executive Officer
CytomX Therapeutics, Inc.
343 Oyster Point Blvd., Suite 100
South San Francisco, CA 94080

Re: CytomX Therapeutics, Inc.
Amendment No. 1 to Registration Statement on Form S -1
Filed September 11, 2015
Response dated September 18, 2015
  File No. 333 -206658

Dear Mr.  McCarthy :

We have reviewed your supplemental response dated September 18, 2015  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 your
response.

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

The Offering, page 8

1. You disclose that the number of shares to be outstanding after the offering is based on
28,259,170 shares of common stock outsta nding as of August 31, 2015 which includes
the conversion of all shares of preferred stock and the net exercise of all outstanding
warrants to purchase shares of preferred stock.  Please tell us how you determined the
amount of common shares to be outstand ing in view of the 27,135,453 converted
preferred shares and 65,884 purchase warrants reflected in footnote 1 to the table on page
11.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 18, 2015
Page 2

Dilution, page 58

2. You state that your historical net tangible book value per share is comprised of your total
tangi ble assets less total liabilities, less preferred stock, divided by the number of your
outstanding shares of common stock.  Please specify the assets that you are excluding
from your total tangible assets and provide your calculations that support your his torical,
pro forma and pro forma as adjusted net tangible book value per share as of June 30,
2015.

Financial Statements

14. Stock Option Plans, page F -29

3. Your disclosures indicate that you granted 697,557 stock options in 2014 with a weighted
average  exercise price of $1.449 and an additional 1,290,705 stock options with a
weighted average exercise price of $2.621 during the six months ended June 30, 2015.
Please address  the following points:

 Provide us with further details for stock and options is sued on dates within the last
year including number of shares/options granted, grant dates, and exercise prices.

 Tell us the methods used and the material assumptions involved in determining the
fair value of the company’s shares in relation to the sto ck or options issued leading up
to the offering.

 To the extent estimated IPO price differs significantly from the most recent fair value
determined for your common shares, please tell us the significant factors and events
contributing to the changes in f air value of the underlying stock leading up to the
expected IPO.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 18, 2015
Page 3

 Please contact Ruairi Regan at (202) 551 -3269 or Pamela Howell, Special Counsel, at
(202) 551 -3357 with any questions.

       Sincerely,

     /s/ Pamela Howell
     for

     John Reynolds
Assistant Director
Office of Beverages, Apparel, and
Mining

cc:  Sam Zucker, Esq.
       Sidley Austin LLP

CORRESP
1
filename1.htm

Response Letter 2

 Sidley Austin LLP

 1001 Page Mill Road

Building 1

 Palo Alto, CA 94304

(650) 565 7000

 (650) 565 7100 FAX

 BEIJING

 BOSTON

BRUSSELS

 CENTURY CITY

CHICAGO

 DALLAS

GENEVA

FOUNDED 1866

 HONG KONG

 HOUSTON

LONDON

 LOS ANGELES

NEW YORK

 PALO ALTO

SAN FRANCISCO

 SHANGHAI

 SINGAPORE

SYDNEY

 TOKYO

WASHINGTON, D.C.

 September 18, 2015

Securities and Exchange Commission

 Division of Corporation
Finance

 100 F Street, N.E.

 Washington, DC 20549

Attn: Ruairi Regan/Pamela Howell

Re:
CytomX Therapeutics, Inc.

Draft Letter Regarding Amendment No. 1 to the

Registration Statement on Form S 1

File No. 333-206658

 Ladies and Gentlemen:

Attached hereto as Exhibit A please find a draft letter responding to comments raised by the staff of the Division of Corporation Finance of
the Securities and Exchange Commission in its letter dated September 17, 2015 regarding Amendment No. 1 to the registration statement on Form S-1 filed by CytomX Therapeutics, Inc. under the Securities Act of 1933, as amended, on
September 11, 2015.

 Please contact me with any comments or questions regarding the attached. The Company would like to begin
marketing the offering on Monday, September 21, 2015, subject of course to the timing of the completion of the Staff’s review. I can be reached at (650) 565-7111. Thank you again for your consideration in this matter.

Very truly yours,

Sam Zucker

 Enclosures

cc:
Robert A. Ryan, Esq., Sidley Austin LLP

Mark V. Roeder, Esq., Latham & Watkins LLP

Brian J. Cuneo, Esq., Latham & Watkins LLP

 Sidley Austin (CA)
LLP is a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in affiliation with other Sidley Austin partnerships.

 EXHIBIT A

 Sidley Austin LLP

 1001 Page Mill Road

Building 1

 Palo Alto, CA 94304

(650) 565 7000

 (650) 565 7100 FAX

 BEIJING

 BOSTON

BRUSSELS

 CENTURY CITY

CHICAGO

 DALLAS

GENEVA

FOUNDED 1866

 HONG KONG

 HOUSTON

LONDON

 LOS ANGELES

NEW YORK

 PALO ALTO

SAN FRANCISCO

 SHANGHAI

 SINGAPORE

SYDNEY

 TOKYO

WASHINGTON, D.C.

 September 21, 2015

Via EDGAR and Courier

 Securities and Exchange Commission

 Division of Corporation Finance

 100 F Street, N.E.

Washington, DC 20549

 Attn: Ruairi Regan/Pamela Howell

Re:
CytomX Therapeutics, Inc.

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

Filed September 11, 2015

File No. 333-206658

 Ladies and Gentlemen:

CytomX Therapeutics, Inc. (the “Company”) has today filed Amendment No. 3 (“Amendment No. 3”) to
the registration statement on Form S-1 (the “Registration Statement”), which Registration Statement was originally filed under the Securities Act of 1933, as amended (the “Securities Act”), on August 28,
2015 and subsequently amended by Amendment No. 1 to the Registration Statement (“Amendment No. 1”), filed on September 11, 2015, and Amendment No. 2 (“Amendment No. 2”), filed on
September 17, 2015. On behalf of the Company, we hereby respond to the comments raised by the staff (the “Staff”) of the Division of Corporation Finance of the Securities and Exchange Commission (the
“Commission”) in regard to the Registration Statement in its letter dated September 17, 2015. For your convenience, the Staff’s comments are included in this letter and are followed by the applicable responses. We will
also provide courtesy copies of Amendment No. 3, as submitted and marked with the changes made from Amendment No. 1.

 Prospectus Cover
Page

1.
 We note the disclosure that certain existing shareholders have indicated an interest to purchase a certain amount of common stock in this offering.
We also note the disclosure that they may purchase more than this amount. So that investors will be able to understand the minimum amount of your shares that will enter the public market through

Sidley Austin (CA) LLP is a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in affiliation with other Sidley Austin partnerships.

 September 21, 2015

  Page
 2

sales to the public, as opposed to the existing shareholders, please disclose whether there is a ceiling on the amount that may be purchased by existing shareholders, and quantify any ceiling.
If not, please add prominent disclosure and discuss the impact on investors where appropriate in the prospectus.

Response: Amendment No. 3 has been revised in response to the Staff’s comment to quantify the indications of interest from the
existing shareholders, and make clear that such indications of interest represent the maximum amount that such shareholder would purchase in the offering. Please see the revisions on the cover page, page 9, pages 44-45, page 59, page 142, page 145
and page 160 of Amendment No. 3.

 Use of Proceeds, page 52

2.
We reissue prior comment 2. We continue to note the broad discretion to change the use of proceeds in the future as discussed in this section and the risk factor on page 44. Please revise to clearly address the
specific alternatives to the use of proceeds based upon the contingencies. See Instruction 7 to Item 504 of Regulation S-K.

Response: Amendment No. 3 has been revised in response to the Staff’s comment to disclose the specific alternatives for the
use of proceeds of the offering based on the contingencies that are most likely to result in the need to reallocate the net proceeds. The specific alternatives to the use of net proceeds specified in Amendment No. 3 reflect the Company’s
expectations in light of the information currently available. These alternatives involve advancing one or more of the Company’s earlier stage programs through preclinical testing and clinical trials, in particular the Company’s T-cell
recruiting bispecific antibodies program and/or PD-1 program, or performing further research and development activities to identify and develop new therapeutics directed against difficult to drug targets. Please see the revisions on pages 53-54 of
Amendment No. 3.

 Business, page 75

Intellectual Property, page 99

In-Licenses, page 102

 License from
UCSB, page 102

3.
Please file the UC licensing agreement as an exhibit.

 Response: The UC licensing
agreement has been filed as Exhibit 10.21 to Amendment No. 2.

 September 21, 2015

  Page
 3

 Executive Compensation, page 129

4.
We reissue prior comment 5. Please provide a more detailed discussion of the specific research and development and financing goals that were used in determining the annual cash bonuses.

Response: Amendment No. 3 has been revised in response to the Staff’s comment to provide more detail with respect to the
research and development and financing goals, including enumerating the strategic goals that applied in the research and development context and specifying certain target financing goals. Please see page 132 to Amendment No. 2.

Exhibits

5.
We reissue prior comment 6. Please file Exhibit 10.15 in its entirety, including all the exhibits, schedules and/or attachments. We note references to Exhibits A and B in the agreement.

 Response: Amendment No. 3 has been revised in response to the Staff’s comment to include Exhibit 10.15 in
its entirety.

*        *        *
 *        *

 September 21, 2015

  Page
 4

 We believe that the proposed modifications to Amendment No. 1, and the supplemental
information contained herein, are responsive to the Staff’s comments. On behalf of the Company, we are available to discuss any of the foregoing at your convenience. Please do not hesitate to contact me at (650) 565-7111 or my colleague,
Robert Ryan, at (212) 839-5931.

Sincerely

Sam Zucker, Esq.

 Enclosures

 cc:

John Reynolds, United States Securities and Exchange Commission

Sean A. McCarthy, D.Phil., President and Chief Executive Officer, CytomX Therapeutics, Inc.

Robert C. Goeltz II, Chief Financial Officer, CytomX Therapeutics, Inc.

Cynthia J. Ladd, Esq., Senior Vice President and General Counsel, CytomX Therapeutics, Inc.

Robert A. Ryan, Esq., Sidley Austin LLP

Mark V. Roeder, Esq., Latham & Watkins LLP

Brian J. Cuneo, Esq., Latham & Watkins LLP

CORRESP
1
filename1.htm

CORRESP

Table of Contents

 Sidley Austin LLP

 1001 Page Mill Road

Building 1

 Palo Alto, CA 94304

(650) 565 7000

 (650) 565 7100 FAX

 BEIJING

 BOSTON

BRUSSELS

 CENTURY CITY

CHICAGO

 DALLAS

GENEVA

FOUNDED 1866

 HONG KONG

 HOUSTON

LONDON

 LOS ANGELES

NEW YORK

 PALO ALTO

SAN FRANCISCO

 SHANGHAI

 SINGAPORE

SYDNEY

 TOKYO

WASHINGTON, D.C.

 September 18, 2015

Securities and Exchange Commission

 Division of Corporation
Finance

 100 F Street, N.E.

 Washington, DC 20549

Attn: Ruairi Regan/Pamela Howell

Re:
CytomX Therapeutics, Inc.

Draft Pages from Amendment No. 3 to the

Registration Statement on Form S-1 (File No. 333-206658)

CIK No. 0001501989

 Ladies and Gentlemen:

Attached hereto please find draft marked pages from Amendment No. 3 to the Registration Statement on Form
S-1 (File No. 333-206658) (the “Registration Statement”) of CytomX Therapeutics, Inc. (the “Company”). The attached pages reflect (i) certain revisions to the
Registration Statement responsive to comments raised by the Staff of the Division of Corporation Finance of the Securities and Exchange Commission in regard to the Registration Statement in the Staff’s letter dated September 17, 2015 and
(ii) certain other revisions, including the share numbers and price range, reverse stock split and related disclosures throughout the filing, including the presentation of Use of Proceeds, Capitalization, Dilution and pro forma and other
financial information.

 Please contact me with any comments or questions regarding the attached. The Company would like to begin marketing
the offering on Monday, September 21, 2015, subject of course to the timing of the completion of the Staff’s review. I can be reached at (650) 565-7111. Thank you again for your consideration in this matter.

Very truly yours,

/s/ Sam Zucker

Sam Zucker

 Enclosures

cc:
Robert A. Ryan, Esq., Sidley Austin LLP
Mark V. Roeder, Esq., Latham & Watkins LLP
Brian J. Cuneo, Esq., Latham & Watkins LLP

 Sidley Austin (CA)
LLP is a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in affiliation with other Sidley Austin partnerships.

Table of Contents

  As filed with the Securities and Exchange Commission on September 21, 2015

Registration Number 333-206658

UNITED STATES

 SECURITIES
AND EXCHANGE COMMISSION

 WASHINGTON, D.C. 20549

  Amendment
No. 3

  to

FORM S-1

 REGISTRATION
STATEMENT

 UNDER

THE SECURITIES ACT OF 1933

 CYTOMX
THERAPEUTICS, INC.

 (Exact name of registrant as specified in its charter)

Delaware

2834

27-3521219

(State or other jurisdiction of incorporation or organization)

(Primary Standard Industrial Classification Code Number)

(I.R.S. Employer Identification Number)

343 Oyster Point Blvd.

Suite 100

 South San
Francisco, CA 94080

 (650) 515-3185

(Address, including zip code, and telephone number, including area code, of registrant’s principal executive offices)

Sean A. McCarthy, D.Phil.

President and Chief Executive Officer

CytomX Therapeutics, Inc.

343 Oyster Point Blvd.

Suite 100

 South San
Francisco, CA 94080

 (650) 515-3185

(Name, address, including zip code, and telephone number, including area code, of agent for service)

With copies to:

 Sam Zucker, Esq.

Robert A. Ryan, Esq.

 Sidley
Austin LLP

 1001 Page Mill Road, Building 1

Palo Alto, CA 94304

Telephone: (650) 565-7000

Fax: (650) 565-7100

 Mark V. Roeder, Esq.

Brian J. Cuneo, Esq.

 Latham
& Watkins LLP

 140 Scott Drive

Menlo Park, CA 94025

Telephone: (650) 328-4600

Fax: (650) 463-2600

 Approximate date of commencement of proposed sale to the public: As soon as practicable after this Registration
Statement becomes effective.

 If any of the securities being registered on this Form are to be offered on a delayed or continuous basis
pursuant to Rule 415 under the Securities Act of 1933 check the following box.  ¨

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the
following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(c) under the Securities Act, check the following box and list the
Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the
Securities Act registration statement number of the earlier effective registration statement for the same offering.  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller
reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one):

Large accelerated filer ¨

Accelerated filer ¨

Non-accelerated filer (Do not check if a smaller reporting company) x

Smaller reporting company ¨

CALCULATION OF REGISTRATION FEE

 Title of each class of

securities to be registered

 Amount

to be

 Registered(1)(2)

 Proposed

Maximum

 Aggregate
Offering

 Price Per Share(2)

 Proposed

Maximum

 Aggregate

Offering Price(1)(2)

 Amount of

Registration Fee(3)

 Common Stock, par value $0.00001 per share

7,187,500

$17.00

$122,187,500.00

$14,198.19

(1)
Includes 937,500 shares of common stock that the underwriters have the option to purchase.

(2)
Estimated solely for the purpose of calculating the amount of the registration fee in accordance with Rule 457(a) under the Securities Act of 1933, as amended.

(3)
The registrant previously paid a total of $11,620.00 in connection with previous filings of the registration statement. In accordance with Rule 457(a), an additional registration fee of $2,578.19 is being paid with this
amendment to the registration statement.

 The Registrant
hereby amends this Registration Statement on such date or dates as may be necessary to delay its effective date until the Registrant shall file a further amendment which specifically states that this Registration Statement shall thereafter
become effective in accordance with Section 8(a) of the Securities Act of 1933 or until the Registration Statement shall become effective on such date as the Commission, acting pursuant to said Section 8(a), may determine.

Table of Contents

 The information contained in this preliminary prospectus is not complete and may be
changed. We may not sell these securities until the registration statement filed with the Securities and Exchange Commission is effective. This preliminary prospectus is not an offer to sell these securities and is not soliciting an offer to buy
these securities in any jurisdiction where the offer or sale is not permitted.

Subject to Completion

Preliminary Prospectus dated September 21, 2015

PROSPECTUS

  6,250,000 Shares

 CytomX Therapeutics, Inc.

Common Stock

  We are
offering shares of our common stock. This is our initial public offering and no public market currently exists for our common stock. We expect the initial public offering price to be between $15.00 and $17.00 per share.

We have applied to list our common stock on The NASDAQ Global Market under the symbol “CTMX.” We are an “emerging growth
company” as defined by the Jumpstart Our Business Startups Act of 2012 and, as such, we have elected to comply with certain reduced public company reporting requirements for this prospectus and future filings.

Investing in our common stock involves a high degree of risk. Please read
“Risk Factors” beginning on page 13 of this prospectus before making a decision to invest in our common stock.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or
determined if this prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

 PER SHARE

 TOTAL

 Public Offering Price

$

$

 Underwriting Discounts and Commissions*

$

$

 Proceeds to us before expenses

$

$

*
We refer you to “Underwriting” beginning on page 157 for additional information regarding underwriting compensation.

Pfizer Inc. (“Pfizer”), an existing stockholder and collaboration partner that is affiliated with one of our directors, has
indicated an interest in purchasing up to $5.0 million in shares of our common stock in this offering. In addition, Bristol-Myers Squibb Company (“BMS”), another of our collaboration partners, has indicated an interest in purchasing
up to $10.0 million in shares of our common stock in this offering. In each case, any shares of our common stock purchased by Pfizer or BMS would be purchased at the initial public offering price and on the same terms as the other purchasers in
this offering. However, because indications of interest are not binding agreements or commitments to purchase, each of Pfizer and BMS may purchase fewer shares than it indicated an interest in purchasing or not purchase any shares in this offering.

 Delivery of the shares of our common stock is expected to be made on or about
                    , 2015. We have granted the underwriters an option for a period of 30 days to purchase an additional 937,500 shares of our
common stock. If the underwriters exercise the option in full, the total underwriting discounts and commissions payable by us will be $                , and the total
proceeds to us, before expenses, will be $            .

 Joint
Book-Running Managers

BofA Merrill Lynch

Jefferies

Cowen and Company

 Co-Manager

Oppenheimer & Co.

 The date of this
prospectus is                     , 2015.

Table of Contents

 TABLE OF CONTENTS

 Explanatory Note

i

 About This Prospectus

ii

 Prospectus Summary

1

 The Offering

8

 Summary Financial and Other Data

10

 Risk Factors

12

 Special Note Regarding Forward-Looking Statements

50

 Statistical Data and Market Information

52

 Use of Proceeds

53

 Dividend Policy

55

 Capitalization

56

 Dilution

58

 Selected Historical Financial Information and Other Data

61

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

63

 Business

77

 Management

121

 Executive and Director Compensation

131

 Certain Relationships and Related Party Transactions

141

 Principal Stockholders

145

 Description of Capital Stock

149

 Shares Eligible for Future Sale

154

 Material U.S. Federal Tax Consequences for Non-U.S. Holders of Common Stock

157

 Underwriting

160

 Legal Matters

167

 Experts

167

 Where You Can Find Additional Information

167

Explanatory Note

Unless we state otherwise or the context otherwise requires, references in this prospectus to:

•

“we,” “our,” “us,” “our company” and “CytomX” refer to CytomX Therapeutics, Inc.;

•

the “FDA” refer to the U.S. Food and Drug Administration;

•

“preferred stock” refer to our redeemable convertible preferred stock and convertible preferred stock;

•

the “JOBS Act” refer to the Jumpstart Our Business Startups Act of 2012;

•

the “Securities Act” refer to the Securities Act of 1933, as amended;

•

the “Exchange Act” refer to the Securities Exchange Act of 1934, as amended; and

•

the “SEC” refer to the Securities and Exchange Commission.

 i

Table of Contents

 PROSPECTUS SUMMARY

This summary highlights selected information contained in greater detail elsewhere in this prospectus and does not contain all of the
information you should consider before investing in our common stock. You should read this entire prospectus carefully, including our financial statements and the related notes included elsewhere in this prospectus. You should also consider, among
other things, the matters described under “Risk Factors” and “Management’s Discussion and Analysis of Financial Condition and Results of Operations,” in each case, appearing elsewhere in this
prospectus.

 Our Company

 Overview

  We are an oncology-focused biopharmaceutical company pioneering a novel class of antibody therapeutics based on our Probody
technology platform. We are using our platform to create proprietary cancer immunotherapies against clinically-validated targets as well as to develop first-in-class cancer therapeutics against novel targets. We believe that our Probody platform
will allow us to improve the combined efficacy and safety profile, or therapeutic window, of monoclonal antibody modalities including cancer immunotherapies, antibody drug conjugates (“ADCs”) and T-cell-recruiting bispecific antibodies.
Our Probody therapeutics are designed to take advantage of unique conditions in the tumor microenvironment to enhance the tumor-targeting features of an antibody and reduce drug activity in healthy tissues. We are currently developing Probody
therapeutics that address clinically-validated cancer targets in immuno-oncology, such as PD-L1, as well as novel targets, such as CD-166, that are difficult to drug and lead to concerns about damage to healthy tissues, or toxicities. In addition to
our proprietary programs, we are collaborating with strategic partners including BMS, Pfizer and ImmunoGen, Inc. (“ImmunoGen”) to develop selected Probody therapeutics. Our broad technology platform and lead product candidates are
supported by a decade of thorough scientific research and strong intellectual property, and we are advancing these candidates toward clinical trials. Our vision is to transform lives with safer, more effective therapies. To realize this vision we
are executing on our mission of changing the treatment of cancer by urgently advancing our Probody pipeline.

  The premise of our
Probody platform is to apply the prodrug concept to create a therapeutic antibody that remains inactive until it reaches the tumor. Probody therapeutics therefore have the potential to produce additional tumor specificity and enhanced safety
profiles because they are designed to have limited interaction with their molecular targets in healthy tissue. This approach of dosing drugs in a form such that they are only activated after reaching certain tissues is called the prodrug approach,
and has been used with many small molecule drugs, but has never before been effectively pursued using therapeutic antibodies.

 Cancer is
the second leading cause of death in the United States, accounting for nearly one in every four deaths. Over the past several decades, cancer research and treatment has evolved from small molecule chemotherapy agents to more targeted monoclonal
antibodies and, more recently, cancer immunotherapies that aim to enhance the ability of the immune system to attack tumors. Despite these advancements, many therapeutic antibodies have the inherent limitation of suboptimal therapeutic window. We
believe that there remains a significant need for therapeutics that are efficacious, safe and tolerable and that our technology represents the next evolution of targeted cancer therapies.

Our Platform

 A Probody
therapeutic consists of three components produced as a single protein by standard antibody production methodology: an active anti-cancer antibody, a mask for the antibody and a protease-cleavable linker. The mask is a peptide designed to disguise
the active binding site of the antibody to prevent the therapeutic from

 1

Table of Contents

 THE OFFERING

 Issuer

CytomX Therapeutics, Inc.

 Common stock offered by us

6,250,000 shares.

 Common stock to be outstanding immediately after this offering

34,509,170 shares (35,446,670 shares if the underwriters exercise in full their option to purchase additional shares of common stock).

 Underwriters’ option to purchase additional shares

We have granted the underwriters a 30-day option to purchase up to 937,500 additional shares at the public offering price less estimated underwritin

Mail Stop 3561

September 17, 2015

Sean A. McCarthy, D.Phil.
President and Chief Executive Officer
CytomX Therapeutics, Inc.
343 Oyster Point Blvd., Suite 100
South San Francisco, CA 94080

Re: CytomX Therapeutics, Inc.
Amendment No. 1 to Registration Statement on Form S -1
Filed September 11, 2015
  File No. 333 -206658

Dear Mr.  McCarthy :

We have reviewed your amended 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 your
response.

After reviewing any amendment to your registration statement and the information you
provide in response  to these comments, we may have additional comments.   Unless we note
otherwise, our references to prior comments are to comments in our September 4, 2015 letter.

Prospectus Cover Page

1. We note the disclosure that certain existing shareholders have indicated an interest to
purchase a certain amount of common stock in this offering. We also note the disclosure
that they may purchase more than this amount.  So that investors will be able to understand
the minimum amount of your shares that will enter t he public market through sales to the
public, as opposed to the existing shareholders, please disclose whether there is a ceiling on
the amount that may be purchased by existing shareholders, and quantify any ceiling.  If
not, please add prominent disclosu re and discuss the impact on investors where appropriate
in the prospectus.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 17, 2015
Page 2

 Use of Proceeds, page 52

2. We reissue prior comment 2.  We continue to note the broad discretion to change the use of
proceeds in the future as discussed in this section and the r isk factor on page 44.  Please
revise to clearly address the specific alternatives to the use of proceeds based upon the
contingencies.  See Instruction 7 to Item 504 of Regulation S -K.

Business, page 75

Intellectual Property, page 99

In-Licenses, page 102

License from UCSB, page 102

3. Please file the UC licensing agreement as an exhibit.

Executive Compensation, page 129

4. We reissue prior comment 5.  Please provide a more detailed discussion of the specific
research and development and financing goals t hat were used in determining the annual
cash bonuses.

Exhibits

5. We reissue prior comment 6.  Please file Exhibit 10.15 in its entirety, including all the
exhibits, schedules and/or attachments.  We note references to Exhibits A and B in the
agreement.

Please contact Ruairi Regan at (202) 551 -3269 or Pamela Howell, Special Counsel, at
(202) 551 -3357 with any questions.

     Sincerely,

     /s/ Pamela Howell
     for

     John Reynolds
Assistant Director
Office of Beverages,
Apparel, and Mining

cc:  Sam Zucker, Esq.
       Sidley Austin LLP

Mail Stop 3561

September 4, 2015

Sean A. McCarthy, D.Phil.
President and Chief Executive Officer
CytomX Therapeutics, Inc.
343 Oyster Point Blvd., Suite 100
South San Francisco, CA 94080

Re: CytomX Therapeutics, Inc.
Registration Statement on Form S -1
Filed August 28, 2015
  File No. 333 -206658

Dear Mr.  McCarthy :

We have reviewed your amended 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 comm ents apply to your facts and
circumstances or do not believe an amendment is appropriate, please tell us why in your
response.

After reviewing any amendment to your registration statement and the information you
provide in response to these comments, we m ay have additional comments.   Unless we note
otherwise, our references to prior comments are to comments in our August 18, 2015 letter.

The Offering, page 8

1. Please update the common stock outstanding after the offering calculation based on the
most recent practicable date.

Use of Proceeds, page 51

2. We reissue prior comment 3.  We continue to note the broad discretion to change the use
of proceeds in the future as discussed in this section and the risk factor on page 44.
Please revise to clearly add ress the contingencies discussed with specificity and the
alternatives.  See Instruction 7 to Item 504 of Regulation S -K.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 4, 2015
Page 2

Employees, page 117

3. We reissue prior comment 7.  Your disclosure should address all employees, not just full
time employees.  Pleas e revise.  See Item 101(c)(xiii) of Regulation S -K.

Executive Compensation , page 127

4. We reissue prior comment 10.  We note the disclosure in footnote one.  The narrative
following the table indicates that the grant date fair value equaled the cash retainer fees.
However, the dollar amounts disclosed in the narrative following the table are not the
same value as that included in the table.  For example, you stat e Dr. Huh’s annual
retainer was $80,000, but the amount included in the table is $160,000.  Please reconcile
or advise.

5. We reissue comment 11.  Please disclose the specific items of corporate performance that
are taken into account in the formula or cri teria in determining the non -equity incentive
award.  See Item 402(o)(5) of Regulation S -K.

Exhibits

6. Please file Exhibits 10.15 and 10.21 in their entirety including the exhibits, schedules
and/or attachments.

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 responsible for the accuracy
and adequacy of the disclosures they have made.

Notwithstanding our comments, in the event you request acceleration of the effectiv e 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 Com mission from taking any action with respect
to the filing;

the action of the Commission or the staff, acting pursuant to delegated authority, in
declaring the filing effective, does not relieve the company from its full responsibility for
the adequacy a nd accuracy of the disclosure in the filing; and

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
September 4, 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 u s to review any amendment prior to the requested effective date of the
registration statement.

Please contact Ruairi Regan at (202) 551 -3269 or Pamela Howell, Special Counsel, at
(202) 551 -3357 with any questions.

     Sincerely,

     /s/ Pamela Howell
     for

     John Reynol ds
Assistant Director
Office of Beverages,
Apparel, and Mining

cc:  Sam Zucker, Esq.
       Sidley Austin LLP

August 18, 2015

Via E -mail
Sean A. McCarthy, D.Phil.
President and Chief Executive Officer
CytomX Therapeutics, Inc.
343 Oyster Point Blvd., Suite 100
South San Francisco, CA 94080

Re: CytomX Therapeutics, Inc.
Draft Registration Statement on Form S -1
Submitted July 24, 2015
  CIK No. 1501989

Dear Mr.  McCarthy :

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 providing the requested information and either submitting
an amended draft registration statement or publicl y 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 r esponse to these  comments  and your
amended draft registration statement or filed registration statement,  we may have  additional
comments.
General
1. 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 Securities Act,
whether or not they retain copies of the communications.
Special Note Regarding Forward -Looking Sta tements, page 48
2. We note your statement that the prospectus includes forward -looking statements under
the Securities Act of 1933 and the Securities Exchange Act of 1934.  Be advised that
Section 27A(b)(1)(C) of the Securities Act and Section 21E(b)(1)(C) o f the Securities
Exchange Act expressly state that the safe harbor for forward looking statements does not

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
August 18, 2015
Page 2

 apply to statements made by companies that are not reporting companies under section
13(a) or 15(d) of the Securities Exchange Act.  Please explain s upplementally or either:
 delete any references to the Private Securities Litigation Reform
Act; or
 make clear, each time you refer to the Litigation Reform Act, that the safe harbor
does not apply to your company
Use of Proceeds, page 51
3. We note the broad discretion to change the use of proceeds  in the futur e as discussed in
this section and the risk factor on page 44.  Please revise to cl early address  the relate d
contingencies and alternatives .  See Instruction 7 to Item 504 of Regulation S -K.
4. We note the disclosure that you may use a portion of the proceeds to make payments of
interest and principal on your outstanding loan with ATEL Ventures.  Please disclose the
amount that may be allocated to these payments.
Preclinical Proof of Concept,  page 86
5. Please tell us the basis for the graphs provided in this section comparing your probody
therapeutic with other available therapeutics .  It is unclear, for example, whether you
used internal or external data to generate the charts and if external please disclose in the
prospectus the source of such data.
In-Licenses, page 101
6. Please disclose the annual royalties that have been paid to date and the extension fees
paid pursuant to the UC agreement.
7. Please disclose the total number of employees.  See I tem 101(c)(xiii) of Regulation S -K.
Management, page 118
8. Please discuss the specific experience, qualifications, attributes or skills that led to the
conclusion that each of the directors individually should serve as a director, as required
by Item 401(e)( 1) of Regulation S -K.
9. Please disclose Dr. Huh’s business experience for the past 5 years.  Please also disclose
the period during which Mr. Goeltz was employed at Amgen.
Executive Compensation , page 127
10. Please discuss in greater detail the compensation arrangements for the directors in 2014.
We note that Dr. Huh and Mr. Gluck received more cash compensation than discussed in

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
August 18, 2015
Page 3

 this section.  In addition, once known, please revise to provide more detail reg arding the
directors’ compensation following the completion of this offering.
11. We note the elements of executive compensation outlined on page 128.  Please provide
an expanded analysis of how you determined the amount of each element to pay as
required by I tem 402(o) of Regulation S -K.  Your disclosure should address the specific
items of corporate performance that are taken into account in setting compensation
policies and making compensation decisions, such as the formula or criteria used in
determining th e non -equity incentive award, and the target research and finance and
financial goals set for the annual cash bonuses.
12. We note the reference to employment and severance and change in control agreements in
this section.  Please file these agreements as ex hibits.
Policies and Procedures for Related Party Transactions, page 133
13. Please disclose the amounts received from Pfizer pursuant to the collaboration agreement
for the periods required by Item 404(a) of Regulation S -K.
14. Please disclose the standards to be  applied pursuant to your policies and procedures as
required by Item 404(b)(1)(ii) of Regulation S -K.
Principal Stockholders, page 134
15. We note your disclosure in this section is provided as of June 30, 2015.  Please provide
this disclosure as of the most recent practicable date.
16. Please disclose the control person(s) for Cytomx Therapeutics Holdings LLC.  Also, if
Mr. Gluck is a control person then he would be deemed to beneficially own all of the
shares held by Cytomx  Therapeutics Holdings LLC, not just a portion.  Please revise or
advise.
Underwriting, page 149
17. On page 1 52 you indicate that “some  of the underwriters and their affiliates have engaged
in, and may in the future  engage in, investment banking and other commercial dealings”
with you or your affiliates.  Please revise to more fully describe these relationships , as
required by Item 508(a) of Regulation S -K.
Undertakings, page II -5
18. Please include the undertakings requ ired by Item s 512(a)(5)(ii) and (a)(6) of Regulation
S-K.

Sean A. McCarthy, D.Phil.
CytomX Therapeutics, Inc.
August 18, 2015
Page 4

 Please contact Ruairi Regan at (202) 551 -3269 or  Pamela Howell, Special Counsel, at
(202) 551 -3357 with any questions.

Sincerely,

 /s/ Pamela Howell
 for

 John Reynolds
Assistant Director
Office of Beverages,
Appa rel and Mining

cc:   Sam Zucker, Esq.
       Sidley Austin LLP