Sierra Oncology (SRRA) DEFA14A Additional proxy soliciting materials

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

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☐	Definitive Proxy Statement
☒	Definitive Additional Materials
☐	Soliciting Material Pursuant to §240.14a-12

SIERRA ONCOLOGY, INC.

(Name of Registrant as Specified In Its Charter)

N/A

(Name of Person(s) Filing Proxy Statement, if other than the Registrant)

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*** Exercise Your Right to Vote *** Important Notice Regarding the Availability of Proxy Materials for the Stockholder Meeting to Be Held on June 9, 2020. Meeting Information SIERRA ONCOLOGY, INC. Meeting Type: Annual Meeting For holders as of: April 15, 2020 Date: June 9, 2020 Time: 10:00 AM PT Location: Meeting live via the Internet-please visit www.virtualshareholdermeeting.com/SRRA2020. You may attend the meeting via the Internet and vote during the meeting. Have the information that is printed in the box marked by the arrow available and follow the instructions. SIERRA ONCOLOGY, INC. C/O2150-885 WEST GEORGIA STREET You are receiving this communication because you hold shares in VANCOUVER, BC V6C 3E8 the company named above. CANADA This is not a ballot. You cannot use this notice to vote these shares. This communication presents only an overview of the more complete proxy materials that are available to you on the Internet. You may view the proxy materials online at www.proxyvote.com or easily request a paper copy (see reverse side). We encourage you to access and review all of the important information contained in the proxy materials before voting. P37626 See proxy the materials reverse and side voting of this instructions notice to obtain . D02494—

Before You Vote How to Access the Proxy Materials Proxy Materials Available to VIEW or RECEIVE: NOTICE AND PROXY STATEMENT FORM10-K How to View Online: Have the information that is printed in the box marked by the arrow    (located on the following page) and visit: www.proxyvote.com. How to Request and Receive a PAPER orE-MAIL Copy: If you want to receive a paper ore-mail copy of these documents, you must request one. There is NO charge for requesting a copy. Please choose one of the following methods to make your request: 1) BY INTERNET: www.proxyvote.com 2) BY TELEPHONE:1-800-579-1639 3) BYE-MAIL*: sendmaterial@proxyvote.com * If requesting materials bye-mail, please send a blanke-mail with the information that is printed in the box marked by the arrow (located on the following page) in the subject line. Requests, instructions and other inquiries sent to thise-mail address will NOT be forwarded to your investment advisor. Please make the request as instructed above on or before May 26, 2020 to facilitate timely delivery. How To Vote Please Choose One of the Following Voting Methods Vote By Internet: Before The Meeting: Go to www.proxyvote.com. Have the information that is printed in the box marked by the arrow P37626 (located on the following page) available and follow the instructions. - During The Meeting: Go to www.virtualshareholdermeeting.com/SRRA2020. Have the information that is printed in the box marked D02495 by the arrow (located on the following page) available and follow the instructions. Vote By Mail: You can vote by mail by requesting a paper copy of the materials, which will include a proxy card.

Voting Items The Board of Directors recommends you vote FOR Proposal 1: 1. Election of Class II Directors Nominees: 01) Andrew Allen, M.D., Ph.D. 02) Mona Ashiya, Ph.D. 03) Andrew Sinclair, Ph.D. The Board of Directors recommends you vote FOR Proposal 2: 2. To ratify the appointment of Deloitte & Touch LLP as the independent registered public accounting firm for the fiscal year ending December 31, 2020. NOTE: In their discretion, the proxies are authorized to vote upon such other business as may properly come before the annual meeting.—P37626 D02496

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