X0306 4 2023-02-15 0 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001863676 Blackman Samuel C. 2000 SIERRA POINT PARKWAY, SUITE 501 BRISBANE CA 94005 0 1 0 0 Chief Medical Officer Common Stock 2023-02-15 4 M 0 1187 A 1242897 D Common Stock 2023-02-15 4 M 0 1437 A 1244334 D Common Stock 2023-02-16 4 S 0 674 19.573 D 1243660 D Common Stock 1000000 I See Footnote Restricted Stock Units 2023-02-15 4 M 0 1187 0.00 D Common Stock 1187 13065 D Restricted Stock Units 2023-02-15 4 M 0 1437 0.00 D Common Stock 1437 21563 D Each Restricted Stock Unit ("RSU") represents a contingent right to receive one share of the Issuer's Common Stock upon settlement for no consideration. The sale of shares is for the sole purpose of covering the Reporting Person's tax liability with respect to the settlement of RSUs. The Reporting Person is the sole manager, and has shared voting and dispositive power with his wife as members. The Reporting Person continues to report beneficial ownership of all of the Issuer's Common Stock held by the 2021 Blackman Family LLC but disclaims beneficial ownership except to the extent of his and his wife's pecuniary interest therein. The RSUs will vest as to 1/16th of the total award in quarterly installments on February 15, May 15, August 15 and November 15, subject to the Reporting Person's provision of service to the Issuer on each vesting date. /s/ Charles N. York II, Attorney-in-Fact 2023-02-17