X0508 4 2025-11-15 0001845337 Day One Biopharmaceuticals, Inc. DAWN 0001863107 Bender Jeremy 1800 SIERRA POINT PARKWAY, SUITE 200 BRISBANE CA 94005 true true false false CHIEF EXECUTIVE OFFICER 0 Common Stock 2025-11-15 4 M 0 3570 A 164935 D Common Stock 2025-11-15 4 M 0 4750 A 169685 D Common Stock 2025-11-15 4 M 0 11687 A 181372 D Common Stock 2025-11-15 4 M 0 11687 A 193059 D Common Stock 2025-11-17 4 S 0 15894 8.9147 D 177165 D Common Stock 14450 I See footnote Common Stock 20204 I See footnote Common Stock 147702 I See footnote Common Stock 18197 I See footnote Common Stock 1408285 I See footnote Restricted Stock Unit (RSU) 2025-11-15 4 M 0 3570 0 D Common Stock 3570 0 D Restricted Stock Unit (RSU) 2025-11-15 4 M 0 4750 0 D Common Stock 4750 19000 D Restricted Stock Unit (RSU) 2025-11-15 4 M 0 11687 0 D Common Stock 11687 93504 D Restricted Stock Unit (RSU) 2025-11-15 4 M 0 11687 0 D Common Stock 11687 140252 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 price reported in Column 4 is a weighted average price. These shares were sold as part of block trades for multiple security holders of the Issuer at prices ranging from $8.545 to $9.305, inclusive. The Reporting Person undertakes to provide to the Issuer, any securityholder 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 herein with regard to the block trade. Represents shares held by The Jeremy Bender 2022 Grantor Retained Annuity Trust under Irrevocable Trust Agreement dated March 29, 2022 of which the Reporting Person is trustee. Represents shares held by The Jeremy Bender 2023 Grantor Retained Annuity Trust dated June 27, 2023. Represents shares held by The Melissa Bender 2022 Grantor Retained Annuity Trust under Irrevocable Trust Agreement dated March 29, 2022 of which the Reporting Person's spouse is trustee. Represents shares held by The Melissa Bender 2023 Grantor Retained Annuity Trust, dated June 27, 2023. Represents shares held by the Bender Revocable Trust dated January 18, 2017, as amended, Jeremy Bender and Melissa C. Bender, Trustees. 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. RSUs do not expire; they either vest or are canceled prior to the vesting date. /s/ Charles N. York II, as Attorney-in-Fact 2025-11-18