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Stock-based Compensation (Tables) | 6 Months Ended
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Jun. 30, 2020
Stock-based Compensation |
Weighted Average Assumptions | The fair value of the stock options granted during the three and six months ended June 30, 2020 and 2019, was estimated based on the following weighted average assumptions: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Three Months Ended June 30, |  | ​ | Six Months Ended June 30, |
​ |  | 2020 |  | 2019 |  |  | 2020 |  | 2019 |
Dividend yield |  | 0 | % | 0 | % |  | 0 | % | 0 | %
Expected volatility |  | 100 | % | 102 | % |  | 100 | % | 103 | %
Risk-free interest rate |  | 0.40 | % | 2.21 | % |  | 0.77 | % | 2.51 | %
Expected term |  | 5.93 years | ​ | 5.60 years | ​ |  | 5.66 years | ​ | 5.60 years | ​
Stock Option Activity | Stock option activity for the six months ended June 30, 2020, was as follows: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | ​ |  | Weighted |  | Weighted |  | Aggregate
​ | ​ | ​ | ​ | Average | ​ | Average | ​ | Intrinsic
​ | ​ | ​ | ​ | Exercise | ​ | Contractual | ​ | Value
​ | ​ | Number of | ​ | Price | ​ | Term | ​ | (in
​ | ​ | shares | ​ | Per Share | ​ | (in years) | ​ | thousands)
Outstanding at beginning of year |  | 1,839,623 | ​ | $ | 20.42 |  | 6.88 |  | $ | 26,829
Granted |  | 187,811 | ​ |  | 21.12 |  |  |  | ​ |
Exercised |  | (2,000) | ​ |  | 4.77 |  |  |  | ​ |
Forfeited, expired or cancelled |  | (17,036) | ​ |  | 147.00 |  |  |  | ​ |
Outstanding at end of period |  | 2,008,398 | ​ | $ | 19.43 |  | 6.75 | ​ | $ | 16,254
Exercisable at end of period |  | 1,604,258 | ​ | $ | 19.70 |  | 6.18 | ​ | $ | 15,169
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,
​ |  | 2020 |  | 2019 |  | 2020 |  | 2019
Research and development | ​ | $ | — | ​ | $ | 59 | ​ | $ | — | ​ | $ | 108
General and administrative | ​ |  | 773 | ​ |  | 952 | ​ |  | 2,561 | ​ |  | 2,631
Total stock-based compensation expense | ​ | $ | 773 | ​ | $ | 1,011 | ​ | $ | 2,561 | ​ | $ | 2,739

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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=116856206&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=116856206&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=116856206&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=116856206&loc=d3e5070-113901 + Details
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