Company: BLLN
Filing Date: 2025-10-17
Form Type: S-1/A
Source: 0001193125-25-242632
Chunk: 308

Company: BillionToOne, Inc.
Filing Date: 2025-10-17
Form: S-1/A
Chunk 308
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 of Rules 144 or 701, 3,846,000 shares of our Class A common stock will be available for sale in the public market as follows (assuming no exercise of outstanding stock options subsequent to June 30, 2025):

| • |     | beginning on the date of this prospectus, all shares of our Class A common stock sold in this offering will be 
 eligible for immediate sale in the public market, unless purchased by our affiliates; and                      |

| • |     | beginning 180 days after the date of this prospectus, 40,169,279 additional shares of Class A common stock (including                                                                                                                           
 4,552,650 shares of Class A common stock issuable upon the conversion of outstanding Class B common stock) will become eligible for sale in the public market, of which 20,997,235 shares will be held by affiliates and subject to the volume, 
 manner of sale, and other restrictions of Rule 144, as described below.                                                                                                                                                                         |

In addition, after this offering, up to 8,930,919 shares of Class A common stock may be issued upon exercise of outstanding stock options as of June 30, 2025, 4,623,888 of which were vested as of such date. Rule 144 In general, under Rule 144 as currently in effect, once we have been subject to public company reporting requirements of Section 13 or Section 15(d) of the Exchange Act for at least 90 days, an eligible stockholder is entitled to sell such shares without complying with the manner of sale, volume limitation, or notice provisions of Rule 144, subject to compliance with the public information requirements of Rule 144. To be an eligible stockholder under Rule 144, such stockholder must not be deemed to have been one of our affiliates for purposes of the Securities Act at any time during the 90 days preceding a sale and must have beneficially owned the shares proposed to be sold for at least six months, including the holding period of any prior owner 229

other than our affiliates. If such a person has beneficially owned the shares proposed to be sold for at least one year, including the holding period of any prior owner other than our affiliates, then such person is entitled to sell such shares without complying with any of the requirements of Rule 144, subject to the expiration of the lock-upagreements and/or market stand-offarrangements described below, which prevent the sale of shares. In general, under Rule