Mona Ashiya - 01 Jul 2022 Form 4 Insider Report for Sierra Oncology, Inc.

Footnote F1

These securities are held of record by OrbiMed Private Investments VII, LP ("OPI VII"). OrbiMed Capital GP VII LLC ("GP VII") is the general partner of OPI VII and OrbiMed Advisors LLC ("OrbiMed Advisors") is the managing member of GP VII. The Reporting Person is an employee of OrbiMed Advisors. By virtue of such relationships, GP VII and OrbiMed Advisors may be deemed to have voting power and investment power over the securities held by OPI VII and as a result, may be deemed to have beneficial ownership over such securities. OrbiMed Advisors exercises voting and investment power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OPI VII.

Footnote F2

Each of the Reporting Person, OrbiMed Advisors, GP VII, OrbiMed Genesis GP LLC ("Genesis GP"), and OrbiMed Capital LLC ("OrbiMed Capital") disclaims beneficial ownership of the securities reported herein for purposes of Rule 16a-1(a) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), except to the extent of its pecuniary interest therein, if any. This report shall not be deemed an admission that any such entity or person is a beneficial owner of such securities for purpose of Section 16 of the Exchange Act, or for any other purpose.

Footnote F3

These securities are held of record by OrbiMed Genesis Master Fund, L.P. ("Genesis"). Genesis GP is the general partner of Genesis and OrbiMed Advisors is the managing member of Genesis GP. By virtue of such relationships, Genesis GP and OrbiMed Advisors may be deemed to have voting power and investment power over the securities held by Genesis and as a result, may be deemed to have beneficial ownership over such securities. OrbiMed Advisors exercises voting and investment power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by Genesis.

Footnote F4

These securities are held of record by OrbiMed Partners Master Fund Limited ("OPM"). OrbiMed Capital is the investment advisor to OPM. OrbiMed Capital is a relying advisor of OrbiMed Advisors. OrbiMed Advisors and OrbiMed Capital exercise voting and investment power through a management committee comprised of Carl L. Gordon, Sven H. Borho, and W. Carter Neild, each of whom disclaims beneficial ownership of the shares held by OPM.

Footnote F5

The option fully vested on November 29, 2020.

Footnote F6

Pursuant to the terms of the merger agreement between issuer, GlaxoSmithKline plc ("GSK") and a subsidiary of GSK, this option was cancelled on the effective date of the merger in exchange for a cash payment equal to, on a per share basis, the offer price of $55.00 less the exercise price.

Footnote F7

Pursuant to an agreement with OrbiMed Advisors and GP VII, the Reporting Person is obligated to transfer any securities issued under any such stock options or other awards, or the economic benefit thereof, to OrbiMed Advisors and GP VII, which will in turn ensure that such securities or economic benefits are provided to OPI VII.

Footnote F8

The option fully vested on June 9, 2021.

Footnote F9

The option fully vested on June 8, 2022.

Footnote F10

Pursuant to the terms of the merger agreement between issuer, GSK and a subsidiary of GSK (the "Merger"), the Company's outstanding warrants will be treated in accordance with their respective terms. At the effective time of the Merger, (1) any of the Company's outstanding Series A warrants will be cancelled and represent only the right to receive an amount in cash, without interest, equal to the Black Scholes Value (as defined in the Series A Warrants) which is calculated under the terms of the Series A warrants to be $45.98 per share of our common stock subject to the Series A warrants.