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Stock-based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2021
Stock-based Compensation |
Weighted Average Assumptions | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended June 30, |  | ​ | Six Months Ended June 30, |
​ |  | 2021 |  | 2020 |  |  | 2021 |  | 2020 |
Dividend yield |  | 0 | % | 0 | % |  | 0 | % | 0 | %
Expected volatility |  | 76 | % | 100 | % |  | 94 | % | 100 | %
Risk-free interest rate |  | 1.06 | % | 0.40 | % |  | 0.77 | % | 0.77 | %
Expected term |  | 6.00 years | ​ | 5.93 years | ​ |  | 5.68 years | ​ | 5.66 years | ​
Stock Option Activity | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted |  | Weighted |  | ​
​ | ​ | ​ | ​ | Average | ​ | Average | ​ | Aggregate
​ | ​ | ​ | ​ | Exercise | ​ | Contractual | ​ | Intrinsic
​ | ​ | Number of | ​ | Price | ​ | Term | ​ | Value
​ | ​ | shares | ​ | Per Share | ​ | (in years) | ​ | (in thousands)
Outstanding at January 1, 2021 | ​ | ​ | 1,827,906 | ​ | $ | 20.66 |  | ​ | 6.31 |  | $ | 51,401
Granted |  | ​ | 176,429 | ​ |  | 37.62 |  | ​ |  |  | ​ |
Exercised |  | ​ | (73,575) | ​ |  | 13.35 |  | ​ |  |  | ​ |
Forfeited, expired or cancelled | ​ | ​ | (57,115) | ​ |  | 71.26 |  | ​ |  |  | ​ |
Outstanding at June 30, 2021 | ​ | ​ | 1,873,645 | ​ | $ | 21.00 |  | ​ | 6.19 | ​ | $ | 33,125
Exercisable at June 30, 2021 | ​ | ​ | 1,535,686 | ​ | $ | 19.37 |  | ​ | 5.58 | ​ | $ | 30,459
Stock-based Compensation Expense | The following table shows total stock-based compensation expense for stock options and ESPP in the condensed consolidated statements of operations and comprehensive loss (in thousands): ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended June 30, | ​ | Six Months Ended June 30,
​ |  | 2021 |  | 2020 |  | 2021 |  | 2020
Research and development | ​ | $ | — | ​ | $ | — | ​ | $ | — | ​ | $ | —
General and administrative | ​ |  | 768 | ​ |  | 773 | ​ |  | 3,666 | ​ |  | 2,561
Total stock-based compensation expense | ​ | $ | 768 | ​ | $ | 773 | ​ | $ | 3,666 | ​ | $ | 2,561

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + 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 (d) -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 50 -Paragraph 2 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + 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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