Day One Reports Second Quarter 2021 Financial Results and Corporate Progress DAY101 Granted Rare Pediatric Disease Designation from FDA DAY101 Granted Orphan Designation from European Commission Dosed the First Patients

Day One Reports Second Quarter 2021 Financial Results and Corporate Progress

DAY101 Granted Rare Pediatric Disease Designation from FDA

DAY101 Granted Orphan Designation from European Commission

Dosed the First Patients in Pivotal FIREFLY-1 Clinical Trial of DAY101 in Patients with pLGG

Successfully Completed $184 Million Upsized Initial Public Offering Providing Funding into the Second Half of 2023

SOUTH SAN FRANCISCO, CA, August 10, 2021 Day One Biopharmaceuticals (Nasdaq: DAWN), a clinical-stage

biopharmaceutical company dedicated to developing and commercializing targeted therapies for patients of all ages with genomically defined cancers, today announced financial results for the second quarter of 2021 and highlighted recent corporate

Day One made significant progress across multiple clinical and corporate initiatives during the second quarter of 2021, including

dosing the first patients in our ongoing FIREFLY-1 pivotal study of DAY101 in pediatric low-grade glioma, said Jeremy Bender, Ph.D., chief executive officer of Day

One. The success of our recent IPO reflects a strong commitment from our investors who, like all of us at Day One, recognize the therapeutic potential of DAY101. Entering the second half of 2021, we remain well positioned to advance our

pipeline through key data readouts with the goal of fulfilling our mission of developing novel medicines to improve the lives of patients of all ages living with cancer.

Corporate Highlights

Second Quarter 2021 Financial Highlights

DAY101 is an investigational, oral, brain-penetrant, highly-selective type II pan-RAF kinase inhibitor designed to

target a key enzyme in the MAPK signaling pathway. Studies have shown DAY101 has high brain distribution and exposure in comparison to other MAPK pathway inhibitors, thus potentially benefiting patients with primary brain tumors or brain metastases

of solid tumors. DAY101 is a type II RAF inhibitor found to selectively inhibit both monomeric and dimeric RAF kinase, which may broaden its potential clinical application to treat an array of RAF-altered

DAY101 has been studied in over 250 patients, and as a monotherapy demonstrated good tolerability and encouraging anti-tumor activity in

pediatric and adult populations with specific MAPK pathway-alterations. In November 2020, Day One announced preliminary results from PNOC014, an ongoing Phase 1 Pacific Pediatric Neuro-Oncology Consortium (PNOC) network study with DAY101

sponsored by the Dana-Farber Cancer Institute. Preliminary results demonstrated that of the eight relapsed pLGG patients in the study with RAF fusions, two patients achieved a complete response by Response Assessment for Neuro-Oncology (RANO),

three had a partial response, two achieved prolonged stable disease, and one experienced progressive disease. DAY101 also demonstrated a tolerable safety profile with the most common side effects being skin rash and hair color changes.

DAY101 has been granted Breakthrough Therapy designation by the U.S. Food and Drug Administration (FDA) for the treatment of patients with pLGG harboring an

activating RAF alteration who require systemic therapy and who have either progressed following prior treatment or who have no satisfactory alternative treatment options. The FDA has also granted Rare Pediatric Disease Designation to DAY101 for the

treatment of low-grade gliomas harboring an activating RAF alteration that disproportionately affects children. In addition, DAY101 has received Orphan Drug designation from the FDA for the treatment of

malignant glioma and orphan designation from the European Commission for the treatment of glioma.

Day One is conducting a pivotal Phase 2 trial (FIREFLY-1) of DAY101 in pediatric, adolescent and young adult patients with pLGG. Day One also plans to study DAY101 alone or in combination with other agents that target key signaling nodes in the MAPK pathway,

such as the Company s MEK inhibitor pimasertib, in patient populations where various RAS and RAF alterations are believed to play an important role in driving disease.

About Day One Biopharmaceuticals

Biopharmaceuticals is a clinical-stage biopharmaceutical company dedicated to developing and commercializing targeted therapies for patients of all ages with genomically defined cancers. Day One was founded to address a critical unmet need: children

with cancer are being left behind in a cancer drug development revolution. Our name was inspired by the The Day One Talk 1 that physicians have with patients and their families about an

initial cancer diagnosis and treatment plan. We aim to re-envision cancer drug development and redefine what s possible for all people living with cancer regardless of age starting from Day One.

Day One partners with leading clinical oncologists, families, and scientists to identify, acquire, and

develop important emerging cancer treatments. The Company s lead product candidate, DAY101, is an oral, highly-selective type II pan-RAF kinase inhibitor, and is being evaluated in a pivotal Phase 2

clinical trial (FIREFLY-1) in pediatric, adolescent and young adult patients with recurrent or progressive low-grade glioma (pLGG). The Company s pipeline also

includes the investigational agent pimasertib, a clinical-stage, oral, small molecule found to selectively inhibit mitogen-activated protein kinase kinases 1 and 2 (MEK). Through Day One and its collaborators, cancer drug development comes of age.

Day One is based in South San Francisco. For more information, please visit www.dayonebio.com.

Cautionary Note Regarding Forward-Looking

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: Day One s plans to develop cancer therapies, expectations from current clinical trials, the execution of the Phase 2 clinical trial for DAY101 as designed, any

expectations about safety, efficacy, timing and ability to complete clinical trials and to obtain regulatory approvals for DAY101 and other candidates in development, and the ability of DAY101 to treat pLGG or related indications.

Statements including words such as believe, plan, continue, expect, will, develop,

signal, potential, or ongoing and statements in the future tense are forward-looking statements. These forward-looking statements involve risks and uncertainties, as well as assumptions, which, if they do not

fully materialize or prove incorrect, could cause our results to differ materially from those expressed or implied by such forward-looking statements.

Forward-looking statements are subject to risks and uncertainties that may cause Day One s actual activities or results to differ significantly from

those expressed in any forward-looking statement, including risks and uncertainties in this press release and other risks set forth in our filings with the Securities and Exchange Commission, including Day One s ability to develop, obtain

regulatory approval for or commercialize any product candidate, Day One s ability to protect intellectual property, the potential impact of the COVID-19 pandemic and the sufficiency of Day One s

cash, cash equivalents and investments to fund its operations. These forward-looking statements speak only as of the date hereof and Day One specifically disclaims any obligation to update these forward-looking statements or reasons why actual

results might differ, whether as a result of new information, future events or otherwise, except as required by law.

1 Jennifer W. Mack and Holcombe E. Grier; Journal of Clinical Oncology 2004 22:3, 563-566

Day One Biopharmaceuticals, Inc.

Consolidated Statements of Operations and Comprehensive Loss

Day One Biopharmaceuticals, Inc.

Selected Consolidated Balance Sheet Data