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Stock-Based Compensation (Tables) | 3 Months Ended
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Mar. 31, 2021
Stock-Based Compensation |
Schedule of total employee, director and non-employee stock-based compensation expense recognized | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended |
​ | ​ | March 31, |
​ |  | 2021 |  | 2020 |
General and administrative | ​ | $ | 2,737 | ​ | $ | 2,291 | ​
Research and development | ​ |  | 5,556 | ​ |  | 4,221 | ​
​ | ​ | $ | 8,293 | ​ | $ | 6,512 | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended | ​
​ | ​ | March 31, | ​
​ | ​ | 2021 |  | 2020 | ​
Stock options | ​ | $ | 6,530 | ​ | $ | 5,882 | ​
ESPP | ​ | ​ | 248 | ​ | ​ | 194 | ​
RSUs | ​ | ​ | 1,515 | ​ | ​ | 436 | ​
​ | ​ | $ | 8,293 | ​ | $ | 6,512 | ​
Summary of stock option activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  |  |  | Weighted |  | Weighted | ​ | ​
​ | ​ | ​ | ​ | Average |  | Average | ​ | ​
​ | ​ | Number of | ​ | Exercise |  | Remaining | ​ | Aggregate
​ | ​ | Shares Subject | ​ | Price |  | Contractual | ​ | Intrinsic
​ | ​ | to Outstanding | ​ | (Per |  | Term | ​ | Value
​ | ​ | Options | ​ | Share) |  | (in years) | ​ | (in thousands)
Balance at December 31, 2020 |  | 7,751,789 | ​ | $ | 26.23 | ​ | ​ | 7.00 | ​ | $ | 134,941
Options granted |  | 1,321,917 | ​ | $ | 43.24 | ​ | ​ | ​ | ​ | ​ | ​
Options forfeited |  | (64,824) | ​ | $ | 33.00 | ​ | ​ | ​ | ​ | ​ | ​
Options exercised |  | (230,701) | ​ | $ | 23.14 | ​ | ​ | ​ | ​ | ​ | ​
Balance at March 31, 2021 |  | 8,778,181 | ​ | $ | 28.82 | ​ | ​ | 7.25 | ​ | $ | 125,395
Exercisable | ​ | 4,963,607 | ​ | $ | 22.73 | ​ | ​ | 5.95 | ​ | $ | 100,938
Schedule of weighted average assumptions used for estimation of fair value of stock options | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Options | ​
​ | ​ | Three Months Ended | ​
​ | ​ | March 31, | ​
​ |  | 2021 |  | 2020 |
Expected term (years) | ​ | 6.2 |  | 6.3 | ​
Expected volatility | ​ | 55.6 | % | 53.9 | %
Risk-free interest rate | ​ | 1.02 | % | 1.71 | %
Expected dividend yield | ​ | — | % | — | %
Schedule of weighted average assumptions used for estimation of fair value of ESPP | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ESPP | ​
​ | ​ | Three Months Ended | ​
​ | ​ | March 31, | ​
​ |  | 2021 |  | 2020 |
Expected term (years) |  | 0.5 - 2.0 |  | 0.5 - 2.0 | ​
Expected volatility | ​ | 50.8 - 66.4 | % | 50.8 | %
Risk-free interest rate |  | 0.09 - 1.65 | % | 1.56 - 1.65 | %
Expected dividend yield | ​ | — | % | — | %
Summary of restricted stock unity activity | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | Restricted | ​ | Average Grant
​ | ​ | Stock | ​ | Date Fair Value
​ | ​ | Units | ​ | (Per unit)
​ | ​ | ​ | ​ | ​ | ​
Unvested RSUs at December 31, 2020 |  | 358,825 | ​ | $ | 33.04
Granted |  | 247,050 | ​ | ​ | 43.11
Vested |  | (117,808) | ​ | ​ | 31.75
Forfeited |  | (9,071) | ​ | ​ | 32.36
Unvested RSUs at March 31, 2021 |  | 478,996 | ​ | $ | 38.56

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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (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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