X0306 4 2017-04-12 0 0001501989 CytomX Therapeutics, Inc. CTMX 0001192603 STARR KEVIN P 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 0 0 1 0 Common Stock 2017-04-12 4 S 0 28357 16.3555 D 31419 D Common Stock 5170348 I See Footnote These 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"). 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. /s/ Kevin Gillis by power of attorney for Kevin Starr 2017-04-14