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Stock-Based Compensation | 9 Months Ended
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Sep. 30, 2021
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract] |
Stock-Based Compensation | 7. Stock-Based Compensation In January 2021 and 2020, the number of shares of common stock available for issuance under the 2018 Equity Incentive Plan (the "2018 Plan") was increased by approximately 2.0 million and 1.8 million shares, respectively, as a result of the automatic increase provision in the 2018 Plan. Stock Options Stock option activity under the 2018 Plan and 2015 Equity Incentive Plan (the "2015 Plan") is summarized as follows:
|  | Number of Options |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Contractual Term (in years) |  |  | Aggregate Intrinsic Value (in thousands) |
Outstanding at December 31, 2020 |  |  | 6,897,276 |  |  | $ | 24.52 |  |  |  | 8.07 |  |  | $ | 841,704 |
Granted |  |  | 1,478,994 |  |  | $ | 94.80 |  |  |  |  |  |  |
Exercised |  |  | (348,420 | ) |  | $ | 17.06 |  |  |  |  |  |  |
Forfeited or canceled |  |  | (228,118 | ) |  | $ | 41.86 |  |  |  |  |  |  |
Outstanding at September 30, 2021 |  |  | 7,799,732 |  |  | $ | 37.62 |  |  |  | 7.74 |  |  | $ | 431,288 |
Restricted Shares Restricted share activity, including restricted stock awards, restricted stock units, and performance-based restricted stock units, under the 2018 Plan and 2015 Plan is summarized as follows:
|  | Number of Restricted Shares |  |  | Weighted Average Grant Date Fair Value |
Unvested at December 31, 2020 |  |  | 359,945 |  |  | $ | 59.54 |
Granted |  |  | 170,150 |  |  | $ | 90.75 |
Vested |  |  | (83,775 | ) |  | $ | 57.98 |
Canceled |  |  | (34,033 | ) |  | $ | 61.24 |
Unvested at September 30, 2021 |  |  | 412,287 |  |  | $ | 72.59 |
Performance-Based Stock Options and Restricted Stock Units The Company granted 669,581 performance-based stock options during the nine months ended September 30, 2021 (the "2021 PSAs"). The Company did no t grant any performance-based equity awards during 2020. The Company granted 170,150 performance-based options and 128,900 performance-based restricted stock units to employees in 2019 (the "2019 PSAs"). The performance-based equity awards granted will vest one-quarter upon the achievement of specific clinical development milestones. The remaining shares will then vest ratably over three years thereafter. Performance-based stock options and performance-based restricted stock units are recorded as expense beginning when vesting events are determined to be probable. The milestone for the 2019 PSAs was achieved during the second quarter of 2021. As of September 30, 2021, 42,536 performance-based stock options and 28,905 performance-based restricted stock units granted during 2019 have vested. The Company believes that the achievement of the requisite performance condition for 2021 PSAs granted in the first quarter of 2021 continues to be probable. Stock-based compensation expense recognized was $2.1 million and $8.3 million during the three and nine months ended September 30, 2021, respectively, and $1.8 million and $5.5 million during the three and nine months ended September 30, 2020, respectively. 2018 Employee Share Purchase Plan In August 2018, the Company adopted the 2018 Employee Share Purchase Plan (“ESPP”), which became effective on the business day prior to the effectiveness of the registration statement relating to the Company's initial public offering. A total of 460,000 shares of common stock were initially reserved for issuance under the ESPP. The initial offering period of the ESPP was authorized by the Company’s board of directors and commenced on January 4, 2021. Each offering period is approximately twelve months long, with two purchase periods. ESPP participants will purchase shares of common stock at a price per share equal to 85% of the lesser of (1) the fair market value per share of the common stock on the enrollment date or (2) the fair market value of the common stock on the exercise date. The Company issued no shares and 4,295 shares under the ESPP during the three and nine months ended September 30, 2021, respectively. The stock-based compensation expense related to the ESPP was less than $0.1 million and $0.2 million during the three and nine months ended September 30, 2021, respectively. Stock-Based Compensation Expense Stock-based compensation for options, restricted shares, and ESPP is classified in the condensed consolidated statements of operations and comprehensive loss as follows (in thousands):
|  | Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
|  | 2021 |  |  | 2020 |  |  | 2021 |  |  | 2020 |
Research and development |  | $ | 6,429 |  |  | $ | 4,759 |  |  | $ | 17,598 |  |  | $ | 11,983 |
General and administrative |  |  | 5,948 |  |  |  | 3,482 |  |  |  | 15,791 |  |  |  | 9,230 |
Total stock-based compensation |  | $ | 12,377 |  |  | $ | 8,241 |  |  | $ | 33,389 |  |  | $ | 21,213 |
As of September 30, 2021, total unrecognized compensation cost related to the unvested share-based awards and ESPP was $122.4 million, which is expected to be recognized over a weighted-average period of 2.8 years.

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- Definition The entire disclosure for share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 + Details
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