X0609 4 2026-10-14 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 2026-10-14 4 M 0 3183 2.80 A 3183 D Class A Common Stock 2026-10-14 4 S 0 3083 98.3198 D 100 D Class A Common Stock 2026-10-14 4 S 0 100 99 D 0 D Class A Common Stock 2026-10-15 4 M 0 5150 2.80 A 5150 D Class A Common Stock 2026-10-15 4 S 0 900 98.4328 D 4250 D Class A Common Stock 2026-10-15 4 S 0 1861 99.8438 D 2389 D Class A Common Stock 2026-10-15 4 S 0 1276 100.6029 D 1113 D Class A Common Stock 2026-10-15 4 S 0 980 101.7871 D 133 D Class A Common Stock 2026-10-15 4 S 0 133 102.7766 D 0 D Class A Common Stock 2026-10-14 4 S 0 2667 98 D 149168 I By spouse Stock Option (right to buy) 2.80 2026-10-14 4 M 0 3183 2.80 D 2031-06-07 Class A Common Stock 3183 473779 D Stock Option (right to buy) 2.80 2026-10-15 4 M 0 5150 2.80 D 2031-06-07 Class A Common Stock 5150 468629 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 $98.000 to $98.990 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 $98.000 to $98.770 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 $99.270 to $100.250 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 $100.270 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 $101.370 to $102.140 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 $102.765 to $102.830 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 2026-10-16