Sec Form 4 Filing - Atay Oguzhan @ BillionToOne, Inc. - 2025-11-07

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 Atay Oguzhan 2. Issuer Name and Ticker or Trading Symbol BillionToOne, Inc. [ BLLN] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director _____ 10% Owner X __ Officer (give title below) _____ Other (specify below) See Remarks (Last) (First) (Middle) C/O BILLIONTOONE, INC., 1035 O'BRIEN DRIVE 3. Date of Earliest Transaction (MM/DD/YY) 11/07/2025 (Street) MENLO PARK, CA94025 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ 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 11/07/2025 C( 2 ) 262,434 A 262,434 D Common Stock 11/07/2025 J( 3 ) 262,434 D 0 D Common Stock 11/07/2025 J( 3 ) 1,965,108 D 0 D Common Stock 11/07/2025 J( 3 ) 200,000 D 0 I By spouse ( 4 ) Class A Common Stock 11/07/2025 J( 3 ) 262,434 A 262,434 D Class A Common Stock 11/07/2025 J( 3 ) 1,965,108 A 2,227,542 D Class A Common Stock 11/07/2025 J( 3 ) 200,000 A 200,000 I By spouse ( 4 ) Class A Common Stock 11/07/2025 J ( 5 ) 2,227,542 D 0 D

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 Series A-1 Convertible Preferred Stock ( 2 ) 11/07/2025 C 262,434 ( 2 ) ( 2 ) Common Stock 262,434 ( 2 ) 0 D Class B Common Stock ( 6 ) 11/07/2025 J( 5 ) 2,227,542 ( 6 ) ( 6 ) Class A Common Stock 2,227,542 $ 0 2,227,542 D Stock Option (right to buy) $ 2.8 11/07/2025 J( 1 ) 640,000 ( 7 ) 06/07/2031 Common Stock 640,000 ( 1 ) 0 D Stock Option (right to buy) $ 2.8 11/07/2025 J( 1 ) 640,000 ( 7 ) 06/07/2031 Class A Common Stock 640,000 ( 1 ) 640,000 D Stock Option (right to buy) $ 20.04 11/07/2025 J( 1 ) 1,003,102 ( 8 ) 06/18/2035 Common Stock 1,003,102 ( 1 ) 0 D Stock Option (right to buy) $ 20.04 11/07/2025 J( 1 ) 1,003,102 ( 8 ) 06/18/2035 Class A Common Stock 1,003,102 ( 1 ) 1,003,102 D

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Atay Oguzhan C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK, CA94025 X See Remarks Signatures /s/ Thomas P. Lynch, Attorney-in-Fact 11/12/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The total represents shares received upon conversion of shares of Series A-1 Convertible Preferred Stock. ( 2 )The reported security converted into the Issuer's Common Stock on a one-for-one basis automatically immediately prior to the closing of the Issuer's initial public offering of Class A common stock (the "Offering") without payment of further consideration. The shares have no expiration date. ( 3 )Pursuant to a reclassification exempt under Rule 16b-7, each share of Common Stock was automatically reclassified into one share of Class A common stock immediately prior to the completion of the Offering. ( 4 )Represents shares held by the Reporting Person's spouse. ( 5 )Following the reclassification of Common Stock into Class A Common Stock, these shares of Class A common stock were exchanged at a 1:1 ratio for shares of Class B common stock in a transaction previously approved by the Issuer's board of directors. ( 6 )Each share of Class B Common Stock is convertible into one share of Class A common stock at the option of the holder. Class B common stock will convert automatically on a one-for-one basis into shares of the Issuer's Class A common stock upon the earliest of (i) seven years from the date of filing of the amended and restated certificate of incorporation, in connection with the Offering and (ii) the date specified by a vote of the holders of Class B common stock representing a majority of the outstanding shares of Class B common stock. ( 7 )The options are fully vested and exercisable. ( 8 )The options are subject to a service-based vesting requirement, which shall be satisfied over a six-year period. The options vest and become exercisable in equal monthly installments commencing on June 11, 2025, subject to the Reporting Person's continuous service with the Issuer through each such vesting date.

Remarks: Chairman and Chief Executive Officer

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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