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Total stock-based compensation related to all of our share-based payments that we recognized for the three and six months ended June 30, 2021 and 2020 were as follows (in thousands):

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During the six months ended June 30, 2021, we granted options to purchase 5,850,481 shares of common stock with a grant-date weighted-average fair value of $2.32 per share, and 1,049,121 options to purchase shares were exercised. As of June 30, 2021, total stock options outstanding was 30,639,026 shares, of which, 2,018,125 shares outstanding are performance-based stock options wherein the achievement of the corresponding corporate-based milestones was not considered as probable. Accordingly, the related grant date fair value for these performance-based stock options of $4.2 million has not been recognized as stock-based compensation expense as of June 30, 2021. The exercise price of stock options granted under our stock plans is equal to the fair market value of the underlying shares on the date of grant. Options become exercisable at varying dates and generally expire 10 years from the date of grant.

The fair value of each option award is estimated on the date of grant using the Black-Scholes option pricing model. We have segregated option awards into the following three homogenous groups for the purposes of determining fair values of options: officers and directors, all other employees, and consultants. We account for forfeitures as they occur.

We determined weighted-average valuation assumptions separately for each of these groups as follows:

The following table summarizes the weighted-average assumptions relating to options granted pursuant to our equity incentive plans for the three and six months ended June 30, 2021 and 2020:

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During the six months ended June 30, 2021, we granted 233,750 restricted stock units with grant-date weighted-average fair value of $3.67 per share. The restricted stock units granted vests over 1 to 2 years, all of which are outstanding as of June 30, 2021.

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As of June 30, 2021, there were approximately $17.4 million of unrecognized stock-based compensation cost which is expected to be recognized over the remaining weighted-average period of 2.11 years, related to time-based stock options, RSUs and performance-based stock options, wherein achievement of the corresponding corporate-based milestones was considered as probable.

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In January 2021, our Board of Directors approved the 825,000 shares increase in available number of shares for future grant under our 2018 Plan, which became effective upon approval by our stockholders during the stockholders annual meeting in May 2021. As of June 30, 2021, there were 10,381,756 shares of common stock available for future grant under our equity incentive plans.

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Employee Stock Purchase Plan

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Our Purchase Plan permits eligible employees to purchase common stock at a discount through payroll deductions during defined offering periods. The price at which the stock is purchased is equal to the lesser of 85% of the fair market value of our common stock on the first day of the offering or 85% of the fair market value of our common stock on the purchase date.

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The fair value of awards granted under our Purchase Plan is estimated on the date of grant using the Black-Scholes option pricing model, which uses weighted-average assumptions. Our Purchase Plan provides for a twenty-four-month offering period comprised of 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.

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We had a “reset” in January 2020 because the fair market value of our stock on December 31, 2019 was lower than the fair market value of our stock on January 1, 2019, the first day of the offering period. Following the “reset” in January 2020, January 1, 2020 was the new first day of the two-year offering period of our ESPP program. We applied modification accounting in accordance with the relevant accounting guidance. The total incremental fair value associated with this “reset” was approximately $753,000 and is being recognized as expense from January 1, 2020 to December 31, 2021. In July 2020, we had another “reset” because the fair market value of our stock on June 30, 2020 was lower than the fair market value of our stock on January 1, 2020. Following the “reset” in July 2020, July 1, 2020 is the new start date of our two-year offering period of our ESPP program. We applied modification accounting in accordance with the relevant accounting guidance. The total incremental fair value associated with this “reset” was approximately $535,000 and is being amortized to expenses from July 1, 2020 to June 30, 2022.

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In January 2021, our Board of Directors approved the 5,500,000 shares increase in the maximum number of shares authorized for issuance under the Purchase Plan, which became effective upon approval by our stockholders during the annual stockholders meeting in May 2021. As of June 30, 2021, there were 5,039,922 shares reserved for future issuance under the Purchase Plan. As of June 30, 2021, unrecognized stock-based compensation cost related to our Purchase Plan amounted to $292,000, which is expected to be recognized over the remaining weighted average period of 0.49 years.