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Equity Plans (Tables) | 9 Months Ended
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Sep. 30, 2021
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, 2020 |  | 4,648,120 |  | $ | 11.87 |  | 7.61 |  | $ | 40.0
Options granted |  | 1,920,540 | ​ | ​ | 29.93 |  | ​ |  | ​ |
Options exercised |  | (308,125) | ​ | ​ | 9.87 |  | ​ |  | ​ | ​
Options forfeited | ​ | (176,580) | ​ | ​ | 16.60 | ​ | ​ | ​ | ​ | ​
Balances at September 30, 2021 |  | 6,083,955 |  | $ | 17.54 |  | 7.58 | ​ | $ | 27.6
Options exercisable – September 30, 2021 | ​ | 3,121,045 |  | $ | 12.89 |  | 6.29 | ​ | $ | 18.7
Options vested and expected to vest – September 30, 2021 | ​ | 6,083,955 | ​ | $ | 17.54 |  | 7.58 | ​ | $ | 27.6
(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 September 30, 2021. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on September 30, 2021.
Schedule of stock-based compensation expense | Total stock-based compensation expense was as follows (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended | ​ | Nine Months Ended
​ | ​ | September 30, | ​ | September 30,
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020
Research and development | ​ | $ | 2,611 | ​ | $ | 1,006 | ​ | $ | 6,241 | ​ | $ | 3,098
General and administrative | ​ |  | 2,164 | ​ |  | 882 | ​ |  | 5,130 | ​ |  | 2,834
Total stock-based compensation expense | ​ | $ | 4,775 | ​ | $ | 1,888 | ​ | $ | 11,371 | ​ | $ | 5,932
Options to purchase common stock |
Black-Scholes option-pricing model assumptions | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended | ​ | Nine Months Ended
​ | ​ | September 30, | ​ | September 30,
​ | ​ | 2021 | ​ | 2020 | ​ | 2021 | ​ | 2020
Expected term (in years) |  | 6.02 - 6.08 |  | 5.27 - 6.08 | ​ | 5.27 - 6.08 |  | 5.27- 6.08
Expected volatility |  | 87.4% - 88.4% | ​ | 84.5% - 85.1% | ​ | 87.4% - 90.2% | ​ | 72.1% - 85.1%
Risk-free interest rate |  | 0.85% - 1.16% | ​ | 0.23% - 0.40% | ​ | 0.11% - 1.16% | ​ | 0.23% - 1.44%
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, 2020 | ​ | 244,545 | ​ | $ | 9.31
Granted | ​ | 302,250 |  | ​ | 24.40
Vested | ​ | (78,165) | ​ |  | 10.13
Forfeited | ​ | (49,970) | ​ | ​ | 18.86
Unvested RSUs at September 30, 2021 | ​ | 418,660 | ​ | $ | 20.07
Performance Stock Units |
Schedule of performance stock unit activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ |  | Shares |  | Fair Value
Unvested PSUs at December 31, 2020 | ​ | — | ​ | ​ | —
Granted |  | 110,500 | ​ | $ | 23.57
Vested | ​ | — | ​ | ​ | —
Forfeited | ​ | (5,000) | ​ | ​ | —
Unvested PSUs at September 30, 2021 | ​ | 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=115993241&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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