SecProbe.io

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

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 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