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Stock-Based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2021
Stock-Based Compensation. |
Stock-Based Compensation Expense | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended |  | Six Months Ended
​ | ​ | June 30, | ​ | June 30,
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020
​ | ​ | (In thousands)
Research and development | ​ | $ | 1,410 | ​ | $ | 1,631 | ​ | $ | 2,890 | ​ | $ | 3,078
Selling, general and administrative | ​ |  | 1,707 | ​ |  | 2,191 | ​ |  | 3,498 | ​ |  | 4,220
Total | ​ | $ | 3,117 | ​ | $ | 3,822 | ​ | $ | 6,388 | ​ | $ | 7,298
​
Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | Three Months Ended | ​ | ​ | Six Months Ended
​ | ​ | June 30, 2021 | ​ | ​ | June 30, 2021
Estimated weighted-average fair value | ​ | $ | 11.29 | ​ | ​ | ​ | $ | 12.79 | ​
Weighted-average assumptions: | ​ |  | ​ | ​ | ​ | ​ |  | ​ | ​
Expected volatility | ​ |  | 81 | % | ​ | ​ |  | 81 | %
Expected life, in years | ​ |  | 6.1 | ​ | ​ | ​ |  | 6.1 | ​
Risk-free interest rate | ​ |  | 0.98 | % | ​ | ​ |  | 0.89 | %
Expected dividend yield | ​ |  | — | % | ​ | ​ |  | — | %
Schedule of Stock Option Activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted- |  | ​ |  | ​ | ​
​ | ​ | ​ | ​ | Average | ​ | ​ | ​ | Aggregate
​ | ​ | ​ | ​ | Exercise | ​ | Remaining | ​ | Intrinsic
​ | ​ | Options | ​ | Price per | ​ | Contractual Life | ​ | Value
​ | ​ | Outstanding | ​ | Share | ​ | (In years) | ​ | (In thousands)
Balance at December 31, 2020 |  | 11,938,528 | ​ | $ | 11.92 |  |  |  | ​ |
Granted |  | 307,450 | ​ |  | 18.53 |  |  |  | ​ |
Exercised |  | (794,808) | ​ |  | 9.40 |  |  |  | ​ |
Forfeited |  | (135,617) | ​ |  | 15.42 |  |  |  | ​ |
Balance at June 30, 2021 |  | 11,315,553 | ​ | $ | 12.24 |  | 5.7 | ​ | $ | 33,730
Vested and expected to vest at June 30, 2021 |  | 11,019,841 | ​ | $ | 12.20 |  | 5.6 | ​ | $ | 33,217
Exercisable at June 30, 2021 |  | 8,719,802 | ​ | $ | 11.82 |  | 4.9 | ​ | $ | 29,075
​

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