X0609 4 2026-09-08 0 0002070849 BillionToOne, Inc. BLLN 0001659824 Lynch Thomas P. false C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK CA 94025 0 1 0 0 See Remarks 1 Class A Common Stock 2026-09-08 4 M 0 11042 8.65 A 11742 D Class A Common Stock 2026-09-08 4 M 0 3969 11.55 A 15711 D Class A Common Stock 2026-09-08 4 M 0 1663 17.12 A 17374 D Class A Common Stock 2026-09-08 4 M 0 3326 30.78 A 20700 D Class A Common Stock 2026-09-08 4 S 0 38 96.4739 D 20662 D Class A Common Stock 2026-09-08 4 S 0 1944 98.9662 D 18718 D Class A Common Stock 2026-09-08 4 S 0 8285 100.1228 D 10433 D Class A Common Stock 2026-09-08 4 S 0 775 100.8289 D 9658 D Class A Common Stock 2026-09-08 4 S 0 614 98.9986 D 9044 D Class A Common Stock 2026-09-08 4 S 0 2997 100.1135 D 6047 D Class A Common Stock 2026-09-08 4 S 0 358 100.8234 D 5689 D Class A Common Stock 2026-09-08 4 S 0 385 99.223 D 5304 D Class A Common Stock 2026-09-08 4 S 0 1131 100.3261 D 4173 D Class A Common Stock 2026-09-08 4 S 0 147 101.4077 D 4026 D Class A Common Stock 2026-09-08 4 S 0 576 98.8882 D 3450 D Class A Common Stock 2026-09-08 4 S 0 2518 100.1959 D 932 D Class A Common Stock 2026-09-08 4 S 0 232 100.9242 D 700 D Stock Option (right to buy) 8.65 2026-09-08 4 M 0 11042 8.65 D 2033-01-18 Class A Common Stock 11042 68358 D Stock Option (right to buy) 11.55 2026-09-08 4 M 0 3969 11.55 D 2033-10-17 Class A Common Stock 3969 11031 D Stock Option (right to buy) 17.12 2026-09-08 4 M 0 1663 17.12 D 2034-10-16 Class A Common Stock 1663 8337 D Stock Option (right to buy) 30.78 2026-09-08 4 M 0 3326 30.78 D 2035-09-30 Class A Common Stock 3326 36674 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 8, 2026. The reported price in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $98.340 to $99.230 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 $99.585 to $100.565 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 $100.620 to $100.9125 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 $98.340 to $99.195 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 $100.720 to $100.9025 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 $98.830 to $99.825 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 $99.830 to $100.790 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 $100.890 to $101.870 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 $99.670 to $100.550 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 $100.725 to $101.650 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 options are fully vested and exercisable. The terms of the underlying option grant are as previously reported on the Reporting Person's Form 3 filed on November 5, 2025. General Counsel, Chief Compliance Officer and Secretary Thomas P. Lynch 2026-09-10