X0306 4 2017-03-23 0 0001501989 CytomX Therapeutics, Inc. CTMX 0001620014 Exter Neil 29 NEWBURY STREET, 3RD FLOOR BOSTON MA 02116 1 0 0 0 Common Stock 2017-03-23 4 S 0 0 0.00 D 0 I See footnote Common Stock 2017-03-27 4 S 0 0 0.00 D 0 I See footnote Common Stock 11199 I By The Neil Exter 2012 Irrevocable On March 23, 2017, Third Rock Ventures, L.P. ("TRV") sold 1,000,000 shares of Common Stock of the Issuer. The shares were sold in one execution at a price per share of $17.0002. After the transaction referenced in note (1), TRV owned 5,820,348 shares of Common Stock of the Issuer. The reporting person is a partner of Third Rock Ventures GP, L.P., 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. On March 27, 2017, TRV sold 650,000 shares of Common Stock of the Issuer. The shares were sold in multiple transactions at prices ranging from $18.00 to $18.45, inclusive, and at a weighted average price of $18.0003. The reporting person 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 (4). After the transaction referenced in note (4), TRV owned 5,170,348 shares of Common Stock of the Issuer. /s/ Neil Exter 2017-03-27