X0407 4 2023-06-05 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 0 Common Stock 2023-06-05 4 S 0 1229 16.00 D 1234363 D Common Stock 1000000 I By the 2021 Blackman Family LLC The sales reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on August 23, 2022. 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. /s/ Charles N. York II, as Attorney-in-Fact 2023-06-07