Company: BLLN
Filing Date: 2025-09-17
Form Type: DRS/A
Source: 0001193125-25-206347
Chunk: 279

Company: BillionToOne, Inc.
Filing Date: 2025-09-17
Form: DRS/A
Chunk 279
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 each of our fiscal years, commencing in 2026 and ending in 2036, by a number equal to the least of:

| • |     | shares; |

| • |     | % of the shares of Class A common stock outstanding on the last business day of the prior fiscal year; or |

| • |     | the number of shares determined by our Board of Directors. |

The number of shares reserved under our ESPP will automatically be adjusted in the event of a stock split, stock dividend or a reverse stock split (including an adjustment to the per-purchaseperiod share limit). Administration The compensation committee of our Board of Directors will administer our ESPP. 207

Eligibility

All of our
employees will be eligible to participate if we employ them for more than 20 hours per week and for five or more months per year. Eligible employees may begin participating in our ESPP at the start of any offering period.

Offering periods

Each offering period will last a number of months
determined by the compensation committee, not to exceed 27 months. A new offering period will begin periodically, as determined by the compensation committee. Offering periods may overlap or may be consecutive. Unless otherwise determined by the
compensation committee, two offering periods of six months’ duration will begin in each year on and . Unless otherwise determined by the administrator, the first offering period will begin
on the effective date of the registration statement related to this offering and will end on , with the first purchase date occurring on .

Amount of contributions

Our ESPP will permit each eligible employee to
purchase Class A common stock through payroll deductions. Each employee’s payroll deductions may not exceed 15% of the employee’s cash compensation. Each participant may purchase up to the number of shares determined by our Board of
Directors on any purchase date, not to exceed 1,500 shares. The value of the shares purchased in any calendar year may not exceed $25,000. Participants may withdraw their contributions at any time before stock is purchased.

Purchase price

The price of each share of Class A common stock
purchased under our ESPP will not be less than 85% of the lower of the fair market value per share of Class A common stock on the first day of the applicable offering period (or, in the case of the first offering period, the price at which one
share of Class A common stock is offered to the public in this offering) or the fair market value per share of Class A common stock on