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Stock-based compensation for the periods presented was as follows (in thousands):

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During the three months ended March 31, 2024, we granted stock options to purchase 5,410,890 shares of common stock with weighted-average grant-date fair value of $0.97 per share, and 90,544 stock options were exercised. The stock options granted during the three months ended March 31, 2024 generally vest over 3 years. As of March 31, 2024, there were 37,135,396 stock options outstanding, of which, 1,322,500 are outstanding performance-based stock options wherein the achievement of the corresponding corporate-based milestones were assessed not probable as of March 31, 2024. Accordingly, none of the $2.5 million grant date fair value for these awards has been recognized as stock-based compensation expense as of March 31, 2024.

The fair value of each option award is estimated on the date of grant using the Black-Scholes option pricing model. The following table summarizes the weighted-average assumptions relating to options granted pursuant to our Equity Incentive Plans (our 2018 Equity Incentive Plan and Inducement Plan, as amended) for the periods presented:

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During the three months ended March 31, 2024, we granted 2,763,979 RSUs with a grant-date weighted-average fair value of $1.27 per share, and 489,992 RSUs were released. The RSUs granted during the three months ended March 31, 2024 generally vest over 3 years. As of March 31, 2024, there were 4,071,854 RSUs outstanding.

As of March 31, 2024, there was approximately $16.2 million of unrecognized stock-based compensation cost which is expected to be recognized over a remaining weighted-average period of 2.49 years, related to time-based stock options, performance-based stock options wherein achievement of the corresponding corporate-based milestones was considered as probable, and RSUs.

In March 2024, our Board of Directors approved additional 375,000 shares of common stock reseved for issuance under our Inducement Plan. As of March 31, 2024, there were 7,129,161 shares of common stock available for future grant under our Equity Incentive Plans.

Employee Stock Purchase Plan

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Our Purchase Plan provides for a 24-month offering period comprises four six-month purchase periods with a look-back option. A look-back option is a provision in our Purchase Plan under which eligible employees can purchase shares of our common stock at a price per share equal to the lesser of 85% of the fair market value on the first day of the offering period or 85% of the fair market value on the purchase date. Our Purchase Plan also includes a feature that provides for a new offering period to begin when the fair market value of our common stock on any purchase date during an offering period falls below the fair market value of our common stock on the first day of such offering period. This feature is called a “reset.” Participants are automatically enrolled in the new offering period.

Our previous 24-month offering period under our Purchase Plan ended on June 30, 2022, and a new 24-month offering period started on July 1, 2022. The fair value of awards under our Purchase Plan is estimated on the date of our new offering period using the Black-Scholes option pricing model, which is being amortized over the requisite service periods. As of March 31, 2024, unrecognized stock-based compensation cost related to our Purchase Plan amounted to $0.1 million, which is expected to be recognized over the remaining weighted average period of 0.24 years.

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During the three months ended March 31, 2024, there were no shares purchased under the Purchase Plan. As of March 31, 2024, there were 2,495,835 shares reserved for future issuance under the Purchase Plan.