Ownership Submission
FORM 4
Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940
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(Print or Type Responses)
1. Name and Address of Reporting Person *
THIRD ROCK VENTURES LP
   2. Issuer Name and Ticker or Trading Symbol
CytomX Therapeutics, Inc. [CTMX] 		5. Relationship of Reporting Person(s) to Issuer
(Check all applicable)
_____ Director __X__ 10% Owner
_____ Officer (give title below) _____ Other (specify below)
(Last)
(First)
(Middle)
29 NEWBURY STREET, 3RD FLOOR		 3. Date of Earliest Transaction (Month/Day/Year)
10/18/2017
(Street)

BOSTON, MA 02116		 4. If Amendment, Date Original Filed(Month/Day/Year)
 6. Individual or Joint/Group Filing(Check Applicable Line)
___ Form filed by One Reporting Person
_X_ Form filed by More than One Reporting Person
(City)
(State)
(Zip)
Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3)		 2. Transaction Date (Month/Day/Year) 2A. Deemed Execution Date, if any (Month/Day/Year) 3. Transaction Code
(Instr. 8)		 4. Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4 and 5)		 5. Amount of Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 3 and 4)		 6. Ownership Form: Direct (D) or Indirect (I)
(Instr. 4)		 7. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V Amount (A) or (D) Price
Common Stock 10/18/2017   J(1)   1,000,000 D (1) 3,320,348 D (2)  
Common Stock               116,927 D (3)  
Common Stock               145,286 D (4)  
Common Stock               85,509 D (5)  

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Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned
(e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security
(Instr. 3)		 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (Month/Day/Year) 3A. Deemed Execution Date, if any (Month/Day/Year) 4. Transaction Code
(Instr. 8)		 5. Number of Derivative Securities Acquired (A) or Disposed of (D)
(Instr. 3, 4, and 5)		 6. Date Exercisable and Expiration Date
(Month/Day/Year)		 7. Title and Amount of Underlying Securities
(Instr. 3 and 4)		 8. Price of Derivative Security
(Instr. 5)		 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s)
(Instr. 4)		 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 4)		 11. Nature of Indirect Beneficial Ownership
(Instr. 4)
Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares

Reporting Owners

Reporting Owner Name / Address Relationships
 Director  10% Owner  Officer  Other
THIRD ROCK VENTURES LP
29 NEWBURY STREET, 3RD FLOOR
BOSTON, MA 02116		     X    
Third Rock Ventures GP, L.P.
29 NEWBURY STREET, 3RD FLOOR
BOSTON, MA 02116		     X    
TRV GP, LLC
29 NEWBURY STREET, 3RD FLOOR
BOSTON, MA 02116		     X    
LEVIN MARK J
29 NEWBURY STREET, 3RD FLOOR
BOSTON, MA 02116		     X    
STARR KEVIN P
29 NEWBURY STREET, 3RD FLOOR
BOSTON, MA 02116		     X    
TEPPER ROBERT I
29 NEWBURY STREET, 3RD FLOOR
BOSTON, MA 02116		     X    

Signatures

 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC, general partner of Third Rock Ventures GP, L.P., general partner of Third Rock Ventures, L.P.   10/19/2017
**Signature of Reporting Person Date
 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC, general partner of Third Rock Ventures GP, L.P.   10/19/2017
**Signature of Reporting Person Date
 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC   10/19/2017
**Signature of Reporting Person Date
 /s/ Kevin Gillis by power of attorney for Mark Levin   10/19/2017
**Signature of Reporting Person Date
 /s/ Kevin Gillis by power of attorney for Kevin Starr   10/19/2017
**Signature of Reporting Person Date
 /s/ Kevin Gillis by power of attorney for Robert I. Tepper   10/19/2017
**Signature of Reporting Person Date

Explanation of Responses:

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1) Pursuant to a Rule 10b5-1 trading plan adopted by Third Rock Ventures, L.P. ("TRV"), TRV distributed on October 18, 2017, for no consideration, 1,000,000 shares of Common Stock of the Issuer (the "Shares") to its limited partners and to Third Rock Ventures GP, L.P. ("TRV GP"), the general partner of TRV, representing each such partner's pro rata interest in such Shares. On the same date, TRV GP distributed, for no consideration, the Shares it received in the distribution by TRV to its partners, representing each such partner's pro rata interest in such Shares. All of the aforementioned distributions were made in accordance with the exemptions afforded by Rules 16a-13 and 16a-9 of the Securities Exchange Act of 1934, as amended.
(2) These shares are directly held by TRV. The general partner of TRV is TRV GP. The general partner of TRV GP is TRV GP, LLC ("TRV GP LLC"). The individual managers of TRV GP LLC are Mark Levin ("Levin"), Kevin Starr ("Starr") and Dr. Robert Tepper ("Tepper"). Each of TRV GP, TRV GP LLC, Levin, Starr and Tepper disclaims beneficial ownership of the shares except to the extent of its or his pecuniary interest therein, if any, and this report shall not be deemed an admission that it or he is the beneficial owner of such shares.
(3) These shares are directly held by Starr. Includes Shares received in the distributions described in footnote (1) above.
(4) These shares are directly held by Levin. Includes Shares received in the distributions described in footnote (1) above.
(5) These shares are directly held by Tepper. Includes Shares received in the distributions described in footnote (1) above.

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