—Company now anticipates MOMENTUM topline data readout by the end of January 2022—

Sierra Oncology, Inc. (NASDAQ: SRRA), a late-stage biopharmaceutical company dedicated to delivering targeted therapies for rare cancers, today announced it has shortened the timeline to the topline data readout of the MOMENTUM pivotal Phase 3 trial of momelotinib in patients with myelofibrosis. The company now expects to announce topline data by the end of January 2022. Assuming positive results, the company intends to file a New Drug Application (NDA) with the US Food & Drug Administration (FDA) in the second quarter of 2022.

“We are really excited at the prospect of seeing topline results from the MOMENTUM study earlier than predicted. This is enabled by the excellent work of the team who have worked extremely hard over recent weeks to ensure quality and completeness of the study data,” said Stephen Dilly, MBBS, PhD, President and Chief Executive Officer at Sierra Oncology. “We look forward to sharing topline results in the next few weeks, and if those data are positive, we are committed to delivering this potential medicine for myelofibrosis patients, particularly those with anemia, as expeditiously as possible.”

About Momelotinib

Momelotinib is a selective and orally bioavailable JAK1, JAK2 and ACVR1 / ALK2 inhibitor for the potential treatment of myelofibrosis. Myelofibrosis results from dysregulated JAK-STAT signaling and is characterized by constitutional symptoms, splenomegaly (enlarged spleen) and progressive anemia.

Momelotinib is currently under investigation in the MOMENTUM clinical trial, a global, randomized, double-blind Phase 3 study for symptomatic and anemic myelofibrosis patients. The study enrolled 195 patients based on a planned 180 patients across 21 countries. Topline data are anticipated by the end of January 2022, and assuming positive results, the company intends to file an NDA with the FDA in the second quarter of 2022.

About Sierra Oncology

Sierra Oncology is a late-stage biopharmaceutical company on a mission to deliver targeted therapies that treat rare forms of cancer. We harness our deep scientific expertise to identify compounds that target the root cause of disease to advance targeted therapies with assets on the leading edge of cancer biology. Our team takes an evidence-based approach to understand the limitations of current treatments and explore new ways to change the cancer treatment paradigm. Together we are transforming promise into patient impact.

For more information, visit www.SierraOncology.com.

Cautionary Note on Forward-Looking Statements

This press release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995, including, but not limited to, statements regarding the expected timing for top-line data in the MOMENTUM clinical trial and the timing of filing a New Drug Application with the US Food & Drug Administration (FDA). All statements other than statements of historical fact are statements that could be deemed forward-looking statements. These statements are based on management's current expectations and beliefs and are subject to a number of risks, uncertainties and assumptions that could cause actual results to differ materially from those described in the forward-looking statements. Such forward-looking statements are subject to risks and uncertainties, including, among others, the risk that Sierra Oncology may not be able to successfully develop, obtain regulatory approval for and commercialize momelotinib or experience significant delays in doing so, Sierra Oncology may not be able to demonstrate acceptable safety and efficacy of momelotinib, the risk that disruptions and impacts of COVID-19 will be significant and lengthy, Sierra Oncology’s ability to continue as a going concern, Sierra Oncology's cash resources may be insufficient to fund its current operating plans and it may be unable to raise additional capital when needed, Sierra Oncology may be unable to acquire additional assets to build a pipeline of additional product candidates, Sierra Oncology's third-party manufacturers may cause its supply of materials to become limited or interrupted or fail to be of satisfactory quantity or quality, Sierra Oncology may be unable to obtain and enforce intellectual property protection for its technologies and momelotinib and the other factors described under the heading "Risk Factors" set forth in Sierra Oncology's filings with the Securities and Exchange Commission from time to time. Sierra Oncology undertakes no obligation to update the forward-looking statements contained herein or to reflect events or circumstances occurring after the date hereof, other than as may be required by applicable law.

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