FORM 4
[ X ] 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
                                                                                  
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Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940
                      

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 (MM/DD/YYYY)

10/18/2017
(Street)

BOSTON, MA 02116
(City)        (State)        (Zip)
4. If Amendment, Date Original Filed (MM/DD/YYYY)

 
6. Individual or Joint/Group Filing (Check Applicable Line)

___ Form filed by One Reporting Person
_ X _ Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1.Title of Security
(Instr. 3) 		2. Trans. Date 2A. Deemed Execution Date, if any 3. Trans. 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)    1000000   D   (1) 3320348   D   (2)  
Common Stock                  116927   D   (3)  
Common Stock                  145286   D   (4)  
Common Stock                  85509   D   (5)  

Table II - Derivative Securities Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security
(Instr. 3) 		2. Conversion or Exercise Price of Derivative Security 3. Trans. Date 3A. Deemed Execution Date, if any 4. Trans. 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 7. Title and Amount of Securities Underlying Derivative Security
(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

Explanation of Responses:
(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.

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


Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* 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).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.
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