Third Rock Ventures SC 13G/A CytomX Therapeutics, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 1)*

CytomX Therapeutics, Inc.

(Name of Issuer)

COMMON STOCK, $0.00001 PAR VALUE PER SHARE

(Title of Class of Securities)

23284F105

(CUSIP Number)

December 31, 2016

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☐ Rule 13d-1(b)

☐ Rule 13d-1(c)

☒ Rule 13d-1(d)

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

CUSIP No. 23284F105 SCHEDULE 13G Page 2 of 12 Pages

1.	NAMES OF REPORTING PERSONS Third Rock Ventures, L.P.
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 0
		6.	SHARED VOTING POWER 7,670,348
		7.	SOLE DISPOSITIVE POWER 0
		8.	SHARED DISPOSITIVE POWER 7,670,348
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,670,348
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 21.1% (1)
12.	TYPE OF REPORTING PERSON PN

(1) The percent of class was calculated based on 36,402,346 shares of common stock issued and outstanding as of November 1, 2016, as disclosed in the Issuer’s Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 3, 2016.

CUSIP No. 23284F105 SCHEDULE 13G Page 3 of 12 Pages

1.	NAMES OF REPORTING PERSONS Third Rock Ventures GP, L.P.
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 0
		6.	SHARED VOTING POWER 7,670,348
		7.	SOLE DISPOSITIVE POWER 0
		8.	SHARED DISPOSITIVE POWER 7,670,348
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,670,348
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 21.1% (1)
12.	TYPE OF REPORTING PERSON PN

CUSIP No. 23284F105 SCHEDULE 13G Page 4 of 12 Pages

1.	NAMES OF REPORTING PERSONS TRV GP, LLC
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION Delaware
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 0
		6.	SHARED VOTING POWER 7,670,348
		7.	SOLE DISPOSITIVE POWER 0
		8.	SHARED DISPOSITIVE POWER 7,670,348
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,670,348
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 21.1% (1)
12.	TYPE OF REPORTING PERSON OO

CUSIP No. 23284F105 SCHEDULE 13G Page 5 of 12 Pages

1.	NAMES OF REPORTING PERSONS Mark Levin
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION United States
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 28,357
		6.	SHARED VOTING POWER 7,670,348
		7.	SOLE DISPOSITIVE POWER 28,357
		8.	SHARED DISPOSITIVE POWER 7,670,348
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,698,705
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 21.1% (1)
12.	TYPE OF REPORTING PERSON IN

CUSIP No. 23284F105 SCHEDULE 13G Page 6 of 12 Pages

1.	NAMES OF REPORTING PERSONS Kevin P. Starr
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION United States
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 28,357
		6.	SHARED VOTING POWER 7,670,348
		7.	SOLE DISPOSITIVE POWER 28,357
		8.	SHARED DISPOSITIVE POWER 7,670,348
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,698,705
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 21.1% (1)
12.	TYPE OF REPORTING PERSON IN

CUSIP No. 23284F105 SCHEDULE 13G Page 7 of 12 Pages

1.	NAMES OF REPORTING PERSONS Robert I. Tepper
2.	CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP (a)  ☐        (b)  ☒
3.	SEC USE ONLY
4.	CITIZENSHIP OR PLACE OF ORGANIZATION United States
NUMBER OF SHARES BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH		5.	SOLE VOTING POWER 28,358
		6.	SHARED VOTING POWER 7,670,348
		7.	SOLE DISPOSITIVE POWER 28,358
		8.	SHARED DISPOSITIVE POWER 7,670,348
9.	AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON 7,698,706
10.	CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES  ☐
11.	PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9 21.1% (1)
12.	TYPE OF REPORTING PERSON IN

CUSIP No. 23284F105 SCHEDULE 13G Page 8 of 12 Pages

Item 1.		Issuer
	(a)	Name of Issuer:
		CytomX Therapeutics, Inc. (the “Issuer”)
	(b)	Address of Issuer’s Principal Executive Offices:
		151 Oyster Point Blvd, Suite 400 South San Francisco, CA 94080
Item 2.		Filing Person
	(a) – (c)	Name of Persons Filing; Address; Citizenship:
		(i)	Third Rock Ventures, L.P. (“TRV”);
		(ii)	Third Rock Ventures GP, L.P. (“TRV GP”), which is the sole general partner of TRV;
		(iii)	TRV GP, LLC (“TRV GP LLC”), which is the sole general partner of TRV GP;
		(iv)	Mark Levin (“Levin”), a managing member of TRV GP LLC;
		(v)	Kevin P. Starr (“Starr”), a managing member of TRV GP LLC; and
		(vi)	Robert I. Tepper (“Tepper,” and collectively with TRV, TRV GP, TRV GP LLC, Levin and Starr, the “Reporting Persons”), a managing member of TRV GP LLC.
		The address of the principal business office of each of the Reporting Persons is Third Rock Ventures, LLC, 29 Newbury Street, 3rd Floor, Boston, MA 02116.
		Each of TRV and TRV GP is a Delaware limited partnership. TRV GP LLC is a Delaware limited liability company. Levin, Tepper, and Starr are United States citizens.
	(d)	Title of Class of Securities:
		Common stock, $0.00001 par value per share, (the “Common Stock”).
	(e)	CUSIP Number:
		23284F105
Item 3.	If this statement is filed pursuant to Rules 13d-1(b), or 13d-2(b) or (c), check whether the person filing is a:
	(a)	☐	Broker or dealer registered under Section 15 of the Act;
	(b)	☐	Bank as defined in Section 3(a)(6) of the Act;
	(c)	☐	Insurance company as defined in Section 3(a)(19) of the Act;
	(d)	☐	Investment company registered under Section 8 of the Investment Company Act of 1940;
	(e)	☐	An investment adviser in accordance with Rule 13d-1(b)(1)(ii)(E);
	(f)	☐	An employee benefit plan or endowment fund in accordance with Rule 13d-1(b)(1)(ii)(F);
	(g)	☐	A parent holding company or control person in accordance with Rule 13d-1(b)(1)(ii)(G);
	(h)	☐	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
	(i)	☐	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940;
	(j)	☐	A non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J);
	(k)	☐	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K). If filing as a non-U.S. institution in accordance with Rule 240.13d-1(b)(1)(ii)(J), please specify the type of institution:

CUSIP No. 23284F105 SCHEDULE 13G Page 9 of 12 Pages

	If filing as a non-U.S. institution in accordance with §240.13d-1(b)(1)(ii)(J), please specify the type of institution:
Item 4.	Ownership.
	(a) and (b)	Amount beneficially owned:
		(i)	TRV directly owns 7,670,348 shares of Common Stock (the “Shares”), which represents approximately 21.1% of the outstanding shares of Common Stock.
		(ii)	TRV GP is the general partner of TRV and may be deemed to beneficially own the Shares.
		(iii)	TRV GP LLC is the general partner of TRV GP and may be deemed to beneficially own the Shares.
		(iv)	As a managing member of TRV GP LLC, Levin may be deemed to beneficially own the Shares. Additionally, as of December 31, 2016, Levin directly owns 28,357 shares of Common Stock. As a result, Levin may be deemed to beneficially own an aggregate of 7,698,705 shares of Common Stock, which represents approximately 21.1% of the outstanding shares of Common Stock.
		(v)	As a managing member of TRV GP LLC, Tepper may be deemed to beneficially own the Shares. Additionally, as of December 31, 2016, Tepper directly owns 28,358 shares of Common Stock. As a result, Tepper may be deemed to beneficially own an aggregate of 7,698,706 shares of Common Stock, which represents approximately 21.1% of the outstanding shares of Common Stock.
		(vi)	As a managing member of TRV GP LLC, Starr may be deemed to beneficially own the Shares. Additionally, as of December 31, 2016, Starr directly owns 28,357 shares of Common Stock. As a result, Starr may be deemed to beneficially own an aggregate of 7,698,705 shares of Common Stock, which represents approximately 21.1% of the outstanding shares of Common Stock.
	(c)	Number of shares as to which such person has:

	Number of Shares of Common Stock
Reporting Person	(i)		(ii)		(iii)		(iv)
TRV				7,670,348				7,670,348
TRV GP				7,670,348				7,670,348
TRV GP LLC				7,670,348				7,670,348
Levin		28,357		7,670,348		28,357		7,670,348
Starr		28,357		7,670,348		28,357		7,670,348
Tepper		28,358		7,670,348		28,358		7,670,348

(i) Sole power to vote or direct the vote

(ii) Shared power to vote or to direct the vote

CUSIP No. 23284F105 SCHEDULE 13G Page 10 of 12 Pages

(iii) Sole power to dispose or to direct the disposition of

(iv) Shared power to dispose or to direct the disposition of

The percent of class was calculated based on 36,402,346 shares of common stock issued and outstanding as of November 1, 2016, as disclosed in the Issuer’s Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on November 3, 2016.

Item 5. Ownership of Five Percent or Less of a Class.

Not applicable.

Item 6. Ownership of More than Five Percent on Behalf of Another Person.

Not applicable.

Item 7. Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.

Not applicable.

Item 8. Identification and Classification of Members of the Group.

Not applicable.

Item 9. Notice of Dissolution of Group.

Not applicable.

Item 10. Certification.

Not applicable.

CUSIP No. 23284F105 SCHEDULE 13G Page 11 of 12 Pages

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Date: February 13, 2017

THIRD ROCK VENTURES, L.P.
By:	THIRD ROCK VENTURES GP, L.P.,
	General Partner
By:	TRV GP, LLC,
	General Partner
By:	/s/ Kevin Gillis
	Kevin Gillis
	Chief Financial Officer
THIRD ROCK VENTURES GP, L.P.
By:	TRV GP, LLC,
	General Partner
By:	/s/ Kevin Gillis
	Kevin Gillis
	Chief Financial Officer
TRV GP, LLC
By:	/s/ Kevin Gillis
	Kevin Gillis
	Chief Financial Officer
MARK LEVIN
/s/ Kevin Gillis, As attorney-in-fact
Mark Levin
KEVIN P. STARR
/s/ Kevin Gillis, As attorney-in-fact
Kevin P. Starr

CUSIP No. 23284F105 SCHEDULE 13G Page 12 of 12 Pages

ROBERT I. TEPPER

/s/ Kevin Gillis, As attorney-in-fact

Robert I. Tepper