X0306 4 2023-01-20 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001655962 York Charles N II 2000 SIERRA POINT PARKWAY, SUITE 501 BRISBANE CA 94005 0 1 0 0 COO, CFO and Secretary Common Stock 2023-01-20 4 S 0 10000 22.7523 D 268810 D The sales reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $22.48 to $23.08 per share, inclusive. The Reporting Person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. Includes 2,313 shares of Common Stock acquired under the Issuer's employee stock purchase plan on November 15, 2021, and May 13, 2022. /s/ Charles N. York II 2023-01-23