X0609 4 2026-10-12 0 0002070849 BillionToOne, Inc. BLLN 0002088549 Atay Oguzhan false C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK CA 94025 1 1 0 0 Chairman and CEO 1 Class A Common Stock 2026-10-12 4 M 0 16667 2.80 A 16667 D Class A Common Stock 2026-10-12 4 S 0 1961 91.5712 D 14706 D Class A Common Stock 2026-10-12 4 S 0 2553 93.046 D 12153 D Class A Common Stock 2026-10-12 4 S 0 6763 93.647 D 5390 D Class A Common Stock 2026-10-12 4 S 0 3869 94.6133 D 1521 D Class A Common Stock 2026-10-12 4 S 0 1521 96.3924 D 0 D Class A Common Stock 2026-10-12 4 S 0 2667 97.61 D 151835 I By spouse Stock Option (right to buy) 2.80 2026-10-12 4 M 0 16667 2.80 D 2031-06-07 Class A Common Stock 16667 476962 D The transactions reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on June 5, 2026. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $91.250 to $92.170 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $92.2596 to $93.255 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $93.260 to $94.225 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $94.265 to $95.205 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $95.945 to $96.765 per share, inclusive. The holder undertakes to provide, upon written request, full information regarding the number of shares sold at each separate price within the range set forth in this footnote. The transactions reported on this Form 4 were effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person's spouse on June 7, 2026. Represents shares held by the Reporting Person's spouse. The options are fully vested and exercisable. /s/ Thomas P. Lynch, Attorney-in-Fact 2026-10-14