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Equity Plans (Tables) | 3 Months Ended
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Mar. 31, 2022
Schedule of activity under equity incentive plans | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted- | ​ | Weighted- | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | Average | ​ | ​
​ | ​ | ​ | ​ | Exercise | ​ | Remaining | ​ | Aggregate
​ | ​ | Options | ​ | Price Per | ​ | Contractual | ​ | Intrinsic
​ |  | Outstanding |  | Share |  | Life (years) |  | Value (1)
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | (in millions)
Balances at December 31, 2021 |  | 5,890,540 |  | $ | 17.66 |  | 7.47 |  | $ | 102.7
Options granted |  | 1,122,100 | ​ | ​ | 28.85 |  | ​ |  | ​ |
Options exercised |  | (165,488) | ​ | ​ | 11.68 |  | ​ |  | ​ | ​
Options forfeited | ​ | (37,542) | ​ | ​ | 39.04 | ​ | ​ | ​ | ​ | ​
Balances at March 31, 2022 |  | 6,809,610 |  | $ | 19.53 |  | 7.37 | ​ | $ | 46.0
Options exercisable – March 31, 2022 | ​ | 3,349,241 |  | $ | 13.59 |  | 6.05 | ​ | $ | 34.5
Options vested and expected to vest – March 31, 2022 | ​ | 6,809,610 | ​ | $ | 19.53 |  | 7.37 | ​ | $ | 46.0
(1) | The aggregate intrinsic values were calculated as the difference between the exercise price of the options and the closing price of the Company’s common stock on March 31, 2022. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on March 31, 2022.
Schedule of stock-based compensation expense | Total stock-based compensation expense was as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31,
​ |  | 2022 |  | 2021
Research and development | ​ | $ | 3,326 | ​ | $ | 1,475
General and administrative | ​ |  | 2,609 | ​ |  | 1,185
Total stock-based compensation expense | ​ | $ | 5,935 | ​ | $ | 2,660
Options to Purchase Common Stock |
Black-Scholes option-pricing model assumptions | ​
​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended March 31,
​ | ​ | 2022 | ​ | 2021
Expected term (in years) |  | 5.27- 6.08 |  | 5.27- 6.08
Expected volatility |  | 97.1% - 98.2% | ​ | 89.8% - 90.2%
Risk-free interest rate |  | 1.64% - 2.13% | ​ | 0.11% - 0.97%
Dividend yield |  | — |  | —
Restricted stock units |
Schedule of activity under equity incentive plans | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ |  | Shares |  | Fair Value
Unvested RSUs at December 31, 2021 | ​ | 405,972 | ​ | $ | 20.13
Granted | ​ | 312,550 | ​ | ​ | 28.73
Vested | ​ | (104,712) | ​ | ​ | 14.73
Forfeited | ​ | (7,000) | ​ | ​ | 28.73
Unvested RSUs at March 31, 2022 | ​ | 606,810 | ​ | $ | 24.61
Performance Stock Units |
Schedule of performance stock unit activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ |  | Shares |  | Fair Value
Unvested PSUs at December 31, 2021 | ​ | 105,500 | ​ | $ | 23.57
Granted |  | — | ​ | ​ | —
Vested | ​ | — | ​ | ​ | —
Forfeited | ​ | — | ​ | ​ | —
Unvested PSUs at March 31, 2022 | ​ | 105,500 | ​ | $ | 23.57

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- Definition Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=122041274&loc=d3e301413-122809 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (f)(2) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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