Sec Form 3 Filing - Bremner Thomas S @ BillionToOne, Inc. - 2025-11-05

FORM 3 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 Bremner Thomas S 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 _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) 1 NORTH WACKER DRIVE, SUITE 2200 3. Date of Earliest Transaction (MM/DD/YY) 11/05/2025 (Street) CHICAGO, IL60606-2807 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

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 ( 1 ) ( 1 ) ( 1 ) Common Stock 1,352 I See footnotes ( 2 ) ( 9 ) ( 10 ) Series C Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 114,117 I See footnotes ( 2 ) ( 9 ) ( 10 ) Series C-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 16,747 I See footnotes ( 2 ) ( 9 ) ( 10 ) Series D Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 20,224 I See footnotes ( 2 ) ( 9 ) ( 10 ) Series C Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 121,688 I See footnotes ( 3 ) ( 9 ) ( 10 ) Series C-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 17,858 I See footnotes ( 3 ) ( 9 ) ( 10 ) Series D Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 16,165 I See footnotes ( 3 ) ( 9 ) ( 10 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 1,562 I See footnotes ( 4 ) ( 9 ) ( 10 ) Series C Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 131,888 I See footnotes ( 4 ) ( 9 ) ( 10 ) Series C-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 19,355 I See footnotes ( 4 ) ( 9 ) ( 10 ) Series D Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 23,466 I See footnotes ( 4 ) ( 9 ) ( 10 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 1,864 I See footnotes ( 5 ) ( 9 ) ( 10 ) Series C Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 157,404 I See footnotes ( 5 ) ( 9 ) ( 10 ) Series C-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 23,100 I See footnotes ( 5 ) ( 9 ) ( 10 ) Series D Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 87,550 I See footnotes ( 5 ) ( 9 ) ( 10 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 652 I See footnotes ( 6 ) ( 9 ) ( 10 ) Series C Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 55,091 I See footnotes ( 6 ) ( 9 ) ( 10 ) Series C-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 8,084 I See footnotes ( 6 ) ( 9 ) ( 10 ) Series D Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 176,245 I See footnotes ( 6 ) ( 9 ) ( 10 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 5,988 I See footnotes ( 7 ) ( 9 ) ( 10 ) Series C Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 505,570 I See footnotes ( 7 ) ( 9 ) ( 10 ) Series C-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 74,195 I See footnotes ( 7 ) ( 9 ) ( 10 ) Series D Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 89,599 I See footnotes ( 7 ) ( 9 ) ( 10 ) Series A-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 1,078 I See footnotes ( 8 ) ( 9 ) ( 10 ) Series C Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 91,003 I See footnotes ( 8 ) ( 9 ) ( 10 ) Series C-1 Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 13,355 I See footnotes ( 8 ) ( 9 ) ( 10 ) Series D Convertible Preferred Stock ( 1 ) ( 1 ) ( 1 ) Common Stock 122,072 I See footnotes ( 8 ) ( 9 ) ( 10 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Bremner Thomas S 1 NORTH WACKER DRIVE, SUITE 2200 CHICAGO, IL60606-2807 X Signatures /s/ Thomas P. Lynch, Attorney-in-Fact 11/05/2025 Signature of Reporting Person Date

Explanation of Responses:

( 1 )The reported security is convertible into the Issuer's Common Stock on a one-for-one basis at any time at the holder's election and shall automatically convert into Class A common stock immediately prior to the closing of the Issuer's initial public offering of Class A common stock without payment of further consideration. The shares have no expiration date. ( 2 )The reported security is directly held by Adams Street 2019 Direct Growth Equity Fund LP. ( 3 )The reported security is directly held by Adams Street 2020 Direct Growth Equity Fund LP. ( 4 )The reported security is directly held by Adams Street 2021 Direct Growth Equity Fund LP. ( 5 )The reported security is directly held by Adams Street 2022 Direct Growth Equity Fund LP. ( 6 )The reported security is directly held by Adams Street Private Equity Navigator Fund LLC (f/k/a Adams Street Global Private Markets Fund LP). ( 7 )The reported security is directly held by Adams Street Growth Equity Fund VII LP. ( 8 )The reported security is directly held by FCPR GF-Lumyna Private Equity World Fund. ( 9 )Adams Street Partners, LLC is the managing member of the general partner of each of Adams Street 2019 Direct Growth Equity Fund LP, Adams Street 2020 Direct Growth Equity Fund LP, Adams Street 2021 Direct Growth Equity Fund LP, Adams Street 2022 Direct Growth Equity Fund LP, Adams Street Growth Equity Fund VII LP, and Adams Street Private Equity Navigator Fund LLC (f/k/a Adams Street Global Private Markets Fund LP) (collectively, the U.S. Funds) and, as a result, may be deemed to beneficially own the shares held by the U.S. Funds. Furthermore, Adams Street Partners, LLC, is the Sous-Delegataire Financier of the Delegataire Financier of the management company of FCPR GF-Lumyna Private Equity World Fund (Lumyna and together with the U.S. Funds, the Funds) and, as a result, ( 10 )(Continued from footnote 9) may be deemed to beneficially own the shares held by Lumyna. Thomas S. Bremner, Jeffrey T. Diehl, Brian Dudley, Elisha P. Gould, Robin Murray and Fred Wang, each of whom is a partner of Adams Street Partners, LLC (or a subsidiary thereof), may be deemed to have shared voting, investment and dispositive power over the shares held by the Funds. The Reporting Person disclaims beneficial ownership of these shares except to the extent of his pecuniary interest therein.

Remarks: Exhibit 24 - Power of Attorney

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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