X0306 4 2022-01-26 0 0001290149 Sierra Oncology, Inc. SRRA 0001295674 Thomson Mary Christina C/O SIERRA ONCOLOGY, INC. 1820 GATEWAY DRIVE, SUITE 110 SAN MATEO CA 94404 0 1 0 0 General Counsel & Secretary Common Stock 2022-01-26 4 M 0 2000 13.04 A 2000 D Common Stock 2022-01-26 4 S 0 2000 29.00 D 0 D Stock Option (Right to Buy) 13.04 2022-01-26 4 M 0 2000 0.00 A 2030-07-28 Common Stock 2000 0 D Stock Option (Right to Buy) 12.15 2022-01-28 4 A 0 12250 0.00 A 2022-01-28 2030-08-12 Common Stock 12250 12250 D The sales reported in this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on January 21, 2022. The Reporting Person is subject to a Lock-Up Letter Agreement that expires on April 26, 2022. The sale of shares is a permissible exemption under the terms of the Lock-Up Letter Agreement. On July 28, 2021, the option vested as to 25% of the total shares granted on July 28, 2020, with the remaining shares from that grant vesting in equal monthly installments over the following 36 months, with 100% of the total shares vested on July 28, 2024, subject to the Reporting Person's provision of service to the Issuer on each vesting date. On August 12, 2020, the Reporting Person was granted an option to purchase 49,000 shares of common stock. The option vests based on the satisfaction by Issuer of certain performance criteria. The achievement of one of the performance criteria resulted in vesting of the option as to 12,250 shares. /s/ Mary Christina Thomson 2022-01-28