FORM 3
        
UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF 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. Date of Event Requiring Statement (MM/DD/YYYY)
10/7/2015 

3. Issuer Name and Ticker or Trading Symbol

CytomX Therapeutics, Inc. [CTMX]

(Last)        (First)        (Middle)

29 NEWBURY STREET, 3RD FLOOR

4. Relationship of Reporting Person(s) to Issuer (Check all applicable)

_____ Director                            ___ X ___ 10% Owner
_____ Officer (give title below)          _____ Other (specify below)

(Street)

BOSTON, MA 02116       

(City)              (State)              (Zip)
5. 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 Beneficially Owned
1.Title of Security
(Instr. 4) 		2. Amount of Securities Beneficially Owned
(Instr. 4) 		3. Ownership Form: Direct (D) or Indirect (I)
(Instr. 5) 		4. Nature of Indirect Beneficial Ownership
(Instr. 5)

Table II - Derivative Securities Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities)
1. Title of Derivate Security
(Instr. 4) 		2. Date Exercisable and Expiration Date
(MM/DD/YYYY) 		3. Title and Amount of Securities Underlying Derivative Security
(Instr. 4) 		4. Conversion or Exercise Price of Derivative Security 5. Ownership Form of Derivative Security: Direct (D) or Indirect (I)
(Instr. 5) 		6. Nature of Indirect Beneficial Ownership
(Instr. 5)
Date Exercisable Expiration Date Title Amount or Number of Shares
Series B-1 Preferred Stock     (1)   (1) Common Stock   8105314   (2)   (1) D   (3)
 
Series C Preferred Stock     (4)   (4) Common Stock   565037   (2)   (4) D   (3)
 

Explanation of Responses:
( 1)  Each share of the Series B-1 Preferred Stock is convertible into Common Stock on a one-for-one basis automatically upon the closing of the Issuer's initial public offering, and has no expiration date.
( 2)  Amount reflects a 62.997-for-1 reverse stock split effected by the Issuer on October 2, 2015.
( 3)  The shares are directly held by Third Rock Ventures, L.P. ("TRV"). The general partner of TRV is Third Rock Ventures GP, L.P. ("TRV GP LP"). The general partner of TRV GP LP 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") and, as such, each of TRV GP LP and TRV GP LLC, Levin, Starr and Tepper exercises shared voting and investment power over the shares held of record by TRV. Each of the Reporting Persons disclaims beneficial ownership of the shares except to the extent of their pecuniary interest therein, if any.
( 4)  Each share of the Series C Preferred Stock is convertible into Common Stock on a one-for-one basis automatically upon the closing of the Issuer's initial public offering, and has no expiration date.

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. 10/8/2015
** Signature of Reporting Person Date

/s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC, general partner of Third Rock Ventures GP, L.P. 10/8/2015
** Signature of Reporting Person Date

/s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC 10/8/2015
** Signature of Reporting Person Date

/s/ Kevin Gillis by power of attorney for Mark Levin 10/8/2015
** Signature of Reporting Person Date

/s/ Kevin Gillis by power of attorney for Kevin Starr 10/8/2015
** Signature of Reporting Person Date

/s/ Kevin Gillis by power of attorney for Robert I. Tepper 10/8/2015
** 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 5(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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