X0306 4 2017-10-18 1 0001501989 CytomX Therapeutics, Inc. CTMX 0001408591 THIRD ROCK VENTURES LP 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001578976 Third Rock Ventures GP, L.P. 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001578973 TRV GP, LLC 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001180428 LEVIN MARK J 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001192603 STARR KEVIN P 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 0001192595 TEPPER ROBERT I 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 Common Stock 2017-10-18 4 J 0 1000000 D 3320348 D Common Stock 116927 D Common Stock 145286 D Common Stock 85509 D 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. 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. These shares are directly held by Starr. Includes Shares received in the distributions described in footnote (1) above. These shares are directly held by Levin. Includes Shares received in the distributions described in footnote (1) above. These shares are directly held by Tepper. Includes Shares received in the distributions described in footnote (1) above. /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. 2017-10-19 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC, general partner of Third Rock Ventures GP, L.P. 2017-10-19 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC 2017-10-19 /s/ Kevin Gillis by power of attorney for Mark Levin 2017-10-19 /s/ Kevin Gillis by power of attorney for Kevin Starr 2017-10-19 /s/ Kevin Gillis by power of attorney for Robert I. Tepper 2017-10-19