Sec Form 4 Filing - AI Day1 LLC @ Day One Biopharmaceuticals, Inc. - 2026-04-23

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person AI Day1 LLC 2. Issuer Name and Ticker or Trading Symbol Day One Biopharmaceuticals, Inc. [ DAWN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) _____ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O ACCESS INDUSTRIES, INC., 40 WEST 57TH STREET, 28TH FLOOR 3. Date of Earliest Transaction (MM/DD/YY) 04/23/2026 (Street) NEW YORK, NY10019 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) _____ Form filed by One Reporting Person __ X __ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 04/23/2026 U( 1 ) 12,929,322 ( 1 ) D $ 21.5 0 D ( 2 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Pre-Funded Warrant (right to buy) $ 0.0001 04/23/2026 D( 1 ) 827,586 ( 1 ) ( 3 ) ( 3 ) Common Stock 827,586 $ 21.4999 0 D ( 2 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other AI Day1 LLC C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019 X Access Industries Holdings LLC C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019 X ACCESS INDUSTRIES MANAGEMENT, LLC C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019 X Blavatnik Len C/O ACCESS INDUSTRIES, INC. 40 WEST 57TH STREET, 28TH FLOOR NEW YORK, NY10019 X Signatures /s/ Alejandro Moreno for AI Day1 LLC 04/27/2026 Signature of Reporting Person Date /s/ Alejandro Moreno for Access Industries Holdings LLC 04/27/2026 Signature of Reporting Person Date /s/ Alejandro Moreno for Access Industries Management, LLC 04/27/2026 Signature of Reporting Person Date /s/ Alejandro Moreno, as Attorney-in-Fact for Mr. Blavatnik 04/27/2026 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On April 9, 2026, AI Day1 tendered all of its shares of Common Stock pursuant to the tender offer by Servier Detroit Inc. ("Purchaser") to purchase all of the issued and outstanding shares of Common Stock for $21.50 per share in cash, upon the terms and subject to the conditions described in the Offer to Purchase and related Letter of Transmittal filed with the SEC on Schedule TO on March 26, 2026. Such shares were accepted by the Purchaser on April 23, 2026. On April 23, 2026, in connection with the consummation of the merger, each pre-funded warrant held by AI Day1 to purchase Common Stock was converted into the right to receive $21.4999 per warrant in cash. ( 2 )The securities reported are held directly by AI Day1 LLC ("AI Day1") and may be deemed to be beneficially owned by Access Industries Holdings LLC ("AIH"), Access Industries Management, LLC ("AIM") and Len Blavatnik because (i) Mr. Blavatnik controls AIM and AIH, (ii) AIM controls AIH, and (iii) AIH owns all of the voting units of AI Day1. Each of AIH, AIM and Mr. Blavatnik, and each of their affiliated entities and the officers, partners, members and managers thereof, disclaims beneficial ownership of these securities, except to the extent of its or his pecuniary interest therein, and this form shall not be construed as an admission that any such reporting person is the beneficial owner of any of the securities reported on this form. ( 3 )Each pre-funded warrant is exercisable at any time after the date of issuance and the pre-funded warrants do not expire. The pre-funded warrants are not exercisable to the extent that the aggregate number of shares of Common Stock beneficially owned by AI Day1 LLC, including together with its direct or indirect affiliates, any person acting or who could be deemed to be acting as a group together with the holder, and any other persons whose beneficial ownership of the Common Stock would or could be aggregated with the holder's for the purposes of Section 13(d) or Section 16 of the Exchange Act of 1934, as amended, immediately following such exercise would exceed 9.99%; provided, however, that AI Day1 LLC may increase or decrease the beneficial ownership limitation by giving 61 days' notice to the Issuer, but not to exceed any percentage in excess of 19.99%.

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