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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, 2020 and 2019 were as follows (in thousands):

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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.

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

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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, 2020 and 2019:

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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.

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We granted options to purchase 7,374,090 shares of common stock during the six months ended June 30, 2020 with a grant-date weighted-average fair value of $1.39 per share. As of June 30, 2020, we had 776,250 shares of outstanding performance-based stock options wherein the achievement of the corresponding corporate-based milestones was not considered as probable. Accordingly, none of the stock-based compensation expense of $1.2 million has been recognized as expense as of June 30, 2020.

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As of June 30, 2020, there were approximately $14.3 million of unrecognized stock-based compensation cost related to time-based stock options and performance-based stock options, wherein achievement of the corresponding corporate-based milestones was considered as probable.

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At June 30, 2020, there were 11,013,055 shares of common stock available for future grant under our equity incentive plans and 581,675 options to purchase shares were exercised during the six months ended June 30, 2020.

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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. The initial offering period commenced on the effective date of our initial public offering.

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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. We had a “reset” on January 2, 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. We applied modification accounting in accordance with the relevant accounting guidance. The total incremental fair value associated with this Purchase Plan “reset” was approximately $753,000 and is being recognized as expense from January 1, 2020 to December 31, 2021. We also had another “reset” on July 1, 2020 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, the first day of the offering period. We applied modification accounting in accordance with the relevant accounting guidance. The total incremental fair value associated with this Purchase Plan “reset” was approximately $535,000 and is being amortized to expenses from July 1, 2020 to June 30, 2022.

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As of June 30, 2020, there were 235,795 shares reserved for future issuance under the Purchase Plan and there was $1.2 million of unrecognized stock-based compensation cost related to our Purchase Plan. The following table summarizes the weighted-average assumptions related to our Purchase Plan for the six months ended June 30, 2020 and 2019. Expected volatilities for our Purchase Plan are based on the historical volatility of our stock. Expected term represents the weighted-average of the purchase periods within the offering period. The risk-free interest rate for periods within the expected term is based on U.S. Treasury constant maturity rates.

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