X0306 4 2016-11-07 0 0001501989 CytomX Therapeutics, Inc. CTMX 0001620014 Exter Neil 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 1 0 0 0 Common Stock 2016-11-07 4 J 0 0 D 0 D On November 7, 2016, Third Rock Ventures, L.P. ("TRV") distributed, 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. The reporting person is a partner of TRV GP, which is the general partner of TRV. The reporting person disclaims beneficial ownership over the shares held by TRV, except to the extent of his pecuniary interest therein, if any, and this report shall not be deemed an admission that he is the beneficial owner of such shares. /s/ Neil Exter 2016-11-09