X0508 4 2025-11-07 0002070849 BillionToOne, Inc. BLLN 0002088549 Atay Oguzhan C/O BILLIONTOONE, INC. 1035 O'BRIEN DRIVE MENLO PARK CA 94025 true true false false See Remarks false Common Stock 2025-11-07 4 C false 262434 A 262434 D Common Stock 2025-11-07 4 J false 262434 D 0 D Common Stock 2025-11-07 4 J false 1965108 D 0 D Common Stock 2025-11-07 4 J false 200000 D 0 I By spouse Class A Common Stock 2025-11-07 4 J false 262434 A 262434 D Class A Common Stock 2025-11-07 4 J false 1965108 A 2227542 D Class A Common Stock 2025-11-07 4 J false 200000 A 200000 I By spouse Class A Common Stock 2025-11-07 4 J false 2227542 D 0 D Series A-1 Convertible Preferred Stock 2025-11-07 4 C false 262434 D Common Stock 262434 0 D Class B Common Stock 2025-11-07 4 J false 2227542 0.00 A Class A Common Stock 2227542 2227542 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 1003102 D 2035-06-18 Common Stock 1003102 0 D Stock Option (right to buy) 20.04 2025-11-07 4 J false 1003102 A 2035-06-18 Class A Common Stock 1003102 1003102 D The total represents shares received upon conversion of shares of Series A-1 Convertible Preferred Stock. The reported security converted into the Issuer's Common Stock on a one-for-one basis automatically immediately prior to the closing of the Issuer's initial public offering of Class A common stock (the "Offering") without payment of further consideration. The shares have no expiration date. 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 Offering. Represents shares held by the Reporting Person's spouse. Following the reclassification of Common Stock into Class A Common Stock, these 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. 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. Chairman and Chief Executive Officer /s/ Thomas P. Lynch, Attorney-in-Fact 2025-11-12