SCHEDULE 13G 0002088549 XXXXXXXX LIVE Class A common stock, par value $0.00001 per share 12/31/2025 0002070849 BillionToOne, Inc. 090168105 1035 O'Brien Drive Menlo Park CA 94025 Rule 13d-1(d) Oguzhan Atay X1 3006861 200000 3006861 200000 3206861 N 7.3 IN Consists of (i) 2,227,542 shares of Class B common stock of the Issuer (as defined in Item 1(a) below) held of record by Oguzhan Atay (the Reporting Person), (ii) 779,319 shares of Class A common stock underlying stock options exercisable within 60 days of the date of this Statement held by the Reporting Person and (iii) 200,000 shares of Class A common stock held of record by the Reporting Person's spouse. Reporting Person disclaims beneficial ownership of securities held by spouse. Each share of Class B common stock is convertible into one share of Class A common stock at the option of the holder. Excludes 863,783 shares of Class A common stock underlying stock options not exercisable within 60 days of the date of this Statement held by the Reporting Person. Ownership percentage based on 44,222,971 shares, which is the sum of (i) 41,216,110 shares of Class A common stock of the Issuer outstanding as of December 5, 2025, as reported by the Issuer in its quarterly report on Form 10-Q filed with the Securities and Exchange Commission on December 10, 2025, (ii) 2,227,542 shares of Class B common stock (such shares treated as converted into shares of Class A common stock solely for the purposes of computing the percentage of ownership of the Reporting Person pursuant to Rule 13d-3 of the Securities Exchange Act of 1934, as amended), and (iii) 779,319 shares of Class A common stock underlying stock options exercisable within 60 days of the date of this Statement held by the Reporting Person. BillionToOne, Inc. 1035 O'Brien Drive, Menlo Park, CA 94025 Oguzhan Atay The address of the principal business office of the Reporting Person is 1035 O'Brien Drive, Menlo Park, CA 94025. The Reporting Person is a citizen of the United States of America. Y See Row 9 of the cover page for the Reporting Person and the corresponding comments. See Row 11 of the cover page for the Reporting Person and the corresponding comments. See Row 5 of the cover page for the Reporting Person and the corresponding comments. See Row 6 of the cover page for the Reporting Person and the corresponding comments. See Row 7 of the cover page for the Reporting Person and the corresponding comments. See Row 8 of the cover page for the Reporting Person and the corresponding comments. Y Y Y Y Y Y Oguzhan Atay /s/ Thomas Lynch, Attorney-in-fact 02/17/2026