SIERRA ONCOLOGY, INC. (NASDAQ:SRRA) Files An 8-K Entry into a Material Definitive Agreement

Item1.01.

On February 9, 2017, Sierra Oncology, Inc. (Sierra Oncology)
entered into an underwriting agreement (the Underwriting
Agreement) with Jefferies LLC, as representative of several
underwriters named on Schedule A thereto (the Underwriters), to
which Sierra Oncology agreed to issue and sell an aggregate of
19,500,000 shares of its common stock (the Shares) to the
Underwriters (the Offering). The Shares will be sold at a per
share public offering price of $1.35. to the Underwriting
Agreement, Sierra Oncology also granted the Underwriters a 30-day
option to purchase up to an additional 2,925,000 shares of its
common stock. The Underwriting Agreement contains customary
representations and warranties, conditions to closing, market
standoff provisions, termination provisions and indemnification
obligations, including for liabilities under the Securities Act
of 1933, as amended. The Offering will be made to a shelf
registration statement on Form S-3 (File No.333-212793) that was
filed by Sierra Oncology with theSecurities and Exchange
Commission(SEC) onAugust 1, 2016and declared effective by
theSEConAugust 11, 2016. Sierra Oncology expects the Offering to
close on February14, 2017, subject to customary closing
conditions.

Sierra Oncology estimates that net proceeds from the Offering
will be approximately $24.3 million, after deducting underwriting
discounts and commissions and estimated Offering expenses, and
assuming no exercise of the Underwriters option to purchase
additional shares. Sierra Oncology intends to use the net
proceeds from the Offering to fund development of its product
candidates, SRA737 and SRA141, as well as for general corporate
purposes. It may also use a portion of the net proceeds to
acquire or license new product candidates or technology that
could result in other product candidates.

The foregoing description of the Underwriting Agreement does not
purport to be complete and is qualified in its entirety by
reference to the full text of the Underwriting Agreement. A copy
of the Underwriting Agreement is filed with this Current Report
on Form 8-K as Exhibit 1.1 and is incorporated herein by
reference.

A copy of the opinion of Fenwick West LLP, relating to the
validity of the Shares in connection with the Offering, is filed
with this Current Report on Form 8-K as Exhibit5.1.

As reported in Sierra Oncologys preliminary prospectus supplement
filed with the SEC on February8, 2017, based on Sierra Oncologys
current estimates, as of December31, 2016, Sierra Oncology had
approximately $109.0million in cash and cash equivalents. The
actual amounts that the Sierra Oncology will report will be
subject to its financial closing procedures and any final
adjustments that may be made prior to the time its financial
results for the period ended December31, 2016 are finalized.

The information in this Item2.02 shall not be deemed to be filed
for purposes of Section18 of the Securities Exchange Act of 1934,
as amended, or otherwise subject to the liabilities of that
section or Sections11 and 12(a)(2)of the Securities Act of 1933,
as amended.The information contained in this Item2.02 shall not
be incorporated by reference into any registration statement or
other document filed by Sierra Oncology with the SEC, whether
made before or after the date of this Current Report on Form8-K,
regardless of any general incorporation language in such filing
(or any reference to this Current Report on Form8-K generally),
except as shall be expressly set forth by specific reference in
such filing.

On February 9, 2017, Sierra Oncology issued a press release
announcing the pricing of the Offering. A copy of the press
release is filed with this Current Report on Form 8-K as Exhibit
99.1 and is incorporated herein by reference.

2

(d)Exhibits.

Forward-Looking Statements

This Current Report on Form 8-K contains forward-looking
statements within the meaning of the U.S. Private Securities
Litigation Reform Act of 1995 and other federal securities laws.
Any statements contained herein that do not describe historical
facts, including, but not limited to, statements regarding the
anticipated use of proceeds of the Offering and the timing of
completion of the Offering, are forward-looking statements that
involve risks and uncertainties that could cause actual results
to differ materially from those discussed in such forward-looking
statements.Such risks and uncertainties include, among others,
the risks identified in Sierra Oncologys filings with theSEC,
including those described under the heading Risk Factors in the
Sierra Oncologys Annual Report on Form 10-K for the year ended
December31, 2016, the Sierra Oncologys Quarterly Reports on Form
10-Q for the quarters ended March31, 2016, June30, 2016 and
September30, 2016, as well as those discussed in the prospectus
supplement related to the Offering, the accompanying prospectus
to the prospectus supplement related to the Offering, the
documents incorporated by reference herein and therein, any
related free writing prospectus and Sierra Oncologys subsequent
filings with theSEC. Any of these risks and uncertainties could
materially and adversely affect Sierra Oncologys results of
operations, which would, in turn, have a significant and adverse
impact on Sierra Oncology stock price. Sierra Oncology cautions
you not to place undue reliance on any forward-looking
statements, which speak only as of the date they are made. Sierra
Oncology undertakes no obligation to update publicly any
forward-looking statements to reflect new information, events or
circumstances after the date they were made or to reflect the
occurrence of unanticipated events.

3

About SIERRA ONCOLOGY, INC. (NASDAQ:SRRA)

Sierra Oncology, Inc., formerly ProNAi Therapeutics, Inc., is a clinical-stage drug development company. The Company’s lead drug candidate, SRA737, is orally bioavailable small molecule inhibitor of Checkpoint kinase 1 (Chk1), a key cell cycle checkpoint and central regulator of the deoxyribonucleic acid (DNA) Damage Response (DDR) network. SRA737 is being investigated in approximately two Phase I clinical trials in patients with advanced cancer. Sierra Oncology is also advancing SRA141, an orally bioavailable small molecule inhibitor of the cell division cycle 7 kinase (Cdc7) kinase undergoing preclinical development. Sierra is building a range of pipeline of various oncology assets against targets at the edge of cancer biology. The Company’s SRA737 and SRA141 target the DDR network, a scientifically approach with far-reaching potential across oncology. SRA141 is an orally available small molecule inhibitor of Cdc7.

SIERRA ONCOLOGY, INC. (NASDAQ:SRRA) Recent Trading Information

SIERRA ONCOLOGY, INC. (NASDAQ:SRRA) closed its last trading session down -0.07 at 1.35 with 1,462,106 shares trading hands.