X0609 4 2026-09-14 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 2026-09-14 4 M 0 16666 2.80 A 16666 D Class A Common Stock 2026-09-14 4 M 0 8333 2.80 A 24999 D Class A Common Stock 2026-09-14 4 S 0 4465 98.6314 D 20534 D Class A Common Stock 2026-09-14 4 S 0 2456 99.5713 D 18078 D Class A Common Stock 2026-09-14 4 S 0 2531 100.5809 D 15547 D Class A Common Stock 2026-09-14 4 S 0 7214 101.4407 D 8333 D Class A Common Stock 2026-09-14 4 S 0 1680 98.6358 D 6653 D Class A Common Stock 2026-09-14 4 S 0 1500 99.5683 D 5153 D Class A Common Stock 2026-09-14 4 S 0 2010 100.7756 D 3143 D Class A Common Stock 2026-09-14 4 S 0 3143 101.4919 D 0 D Class A Common Stock 2026-09-14 4 S 0 2666 98 D 159834 I By spouse Class A Common Stock 2026-09-14 4 S 0 2666 98 D 157168 I By spouse Stock Option (right to buy) 2.80 2026-09-14 4 M 0 16666 2.80 D 2031-06-07 Class A Common Stock 16666 510295 D Stock Option (right to buy) 2.80 2026-09-14 4 M 0 8333 2.80 D 2031-06-07 Class A Common Stock 8333 501962 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 $98.060 to $99.055 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 $99.060 to $100.055 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 $100.060 to $101.040 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 $101.085 to $101.805 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 $98.140 to $99.010 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 $99.160 to $100.130 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 $100.230 to $101.200 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 $101.250 to $101.820 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 2026-09-16