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
                                                                                  
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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 *

Exter Neil 		2. Issuer Name and Ticker or Trading Symbol

CytomX Therapeutics, Inc. [ CTMX ] 		5. Relationship of Reporting Person(s) to Issuer (Check all applicable)

__ X __ Director                      _____ 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)

3/23/2017
(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)

_ X _ Form filed by One Reporting Person
___ 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   3/23/2017     S    0   (1) D $0.00   (1) 0   (2) I   See footnote   (3)
Common Stock   3/27/2017     S    0   (4) D $0.00   (4) 0   (5) I   See footnote   (3)
Common Stock                  11199   I   By The Neil Exter 2012 Irrevocable  

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)  On March 23, 2017, Third Rock Ventures, L.P. ("TRV") sold 1,000,000 shares of Common Stock of the Issuer. The shares were sold in one execution at a price per share of $17.0002.
( 2)  After the transaction referenced in note (1), TRV owned 5,820,348 shares of Common Stock of the Issuer.
( 3)  The reporting person is a partner of Third Rock Ventures GP, L.P., which is the general partner of TRV. The reporting person disclaims beneficial ownership over the shares held by TRV, except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that he is the beneficial owner of such shares.
( 4)  On March 27, 2017, TRV sold 650,000 shares of Common Stock of the Issuer. The shares were sold in multiple transactions at prices ranging from $18.00 to $18.45, inclusive, and at a weighted average price of $18.0003. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in this footnote (4).
( 5)  After the transaction referenced in note (4), TRV owned 5,170,348 shares of Common Stock of the Issuer.

Reporting Owners
Reporting Owner Name / Address
Relationships
Director 10% Owner Officer Other
Exter Neil
29 NEWBURY STREET, 3RD FLOOR
BOSTON, MA 02116 		X



Signatures
/s/ Neil Exter 3/27/2017
** 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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