X0508 4 2025-11-07 0002070849 BillionToOne, Inc. BLLN 0002087127 Tsao David C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK CA 94025 true true false false Chief Technology Officer false Common Stock 2025-11-07 4 J false 2325108 D 0 D Class A Common Stock 2025-11-07 4 J false 2325108 A 2325108 D Class A Common Stock 2025-11-07 4 J false 2325108 D 0 D Class A Common Stock 2025-11-07 4 P false 1000 60.00 A 1000 D Class B Common Stock 2025-11-07 4 J false 2325108 0.00 A Class A Common Stock 2325108 2325108 D Stock Option (right to buy) 2.8 2025-11-07 4 J false 640000 D 2031-06-07 Common Stock 640000 0 D Stock Option (right to buy) 2.8 2025-11-07 4 J false 640000 A 2031-06-07 Class A Common Stock 640000 640000 D Stock Option (right to buy) 20.04 2025-11-07 4 J false 501551 D 2035-06-18 Common Stock 501551 0 D Stock Option (right to buy) 20.04 2025-11-07 4 J false 501551 A 2035-06-18 Class A Common Stock 501551 501551 D Pursuant to a reclassification exempt under Rule 16b-7, each share of Common Stock was automatically reclassified into one share of Class A common stock immediately prior to the completion of the Issuer's initial public offering of Class A common stock (the "Offering"). Following the reclassification of Common Stock into Class A Common Stock, the shares of Class A common stock were exchanged at a 1:1 ratio for shares of Class B common stock in a transaction previously approved by the Issuer's board of directors. These shares of Class A common stock were purchased by the Reporting Person through a directed share program in connection with the Offering. Each share of Class B Common Stock is convertible into one share of Class A common stock at the option of the holder. Class B common stock will convert automatically on a one-for-one basis into shares of the Issuer's Class A common stock upon the earliest of (i) seven years from the date of filing of the amended and restated certificate of incorporation, in connection with the Offering and (ii) the date specified by a vote of the holders of Class B common stock representing a majority of the outstanding shares of Class B common stock. The options are fully vested and exercisable. The options are subject to a service-based vesting requirement, which shall be satisfied over a six-year period. The options vest and become exercisable in equal monthly installments commencing on June 11, 2025, subject to the Reporting Person's continuous service with the Issuer through each such vesting date. /s/ Thomas P. Lynch, Attorney-in-Fact 2025-11-12