X0306 4 2017-03-23 0 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 2017-03-23 4 S 0 1000000 17.0002 D 5820348 D Common Stock 2017-03-27 4 S 0 650000 18.0003 D 5170348 D Common Stock 59776 D Common Stock 59776 D Common Stock 59777 D 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. This price is a weighted average price. These shares were sold in multiple transactions at prices ranging from $18.00 to $18.45, inclusive. Each of the reporting persons 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 (2). These shares are directly held by Starr. These shares are directly held by Levin. These shares are directly held by Tepper. /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. 2017-03-27 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC, general partner of Third Rock Ventures GP, L.P. 2017-03-27 /s/ Kevin Gillis, Chief Financial Officer of TRV GP, LLC 2017-03-27 /s/ Kevin Gillis by power of attorney for Mark Levin 2017-03-27 /s/ Kevin Gillis by power of attorney for Kevin Starr 2017-03-27 /s/ Kevin Gillis by power of attorney for Robert I. Tepper 2017-03-27