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Equity Plans (Tables) | 12 Months Ended
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Dec. 31, 2020
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, 2019 |  | 3,681,521 |  | $ | 11.64 |  | 7.78 |  | $ | 2.4
Options granted |  | 1,536,400 |  |  | 11.53 |  |  |  | ​ |
Options exercised |  | (294,238) | ​ |  | 7.49 |  | ​ |  | ​ | ​
Options forfeited | ​ | (275,563) | ​ | ​ | 11.54 | ​ | ​ | ​ | ​ | ​
Balances at December 31, 2020 |  | 4,648,120 |  | $ | 11.87 |  | 7.61 | ​ | $ | 40.0
Options exercisable – December 31, 2020 | ​ | 2,619,943 |  | $ | 12.10 |  | 6.65 | ​ | $ | 22.1
Options vested and expected to vest – December 31, 2020 | ​ | 4,648,120 | ​ | $ | 11.87 |  | 7.61 | ​ | $ | 40.0
____________________ (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 December 31, 2020. The calculation excludes options with an exercise price higher than the closing price of the Company’s common stock on December 31, 2020. ​
Schedule of stock-based compensation expense | Total stock-based compensation expense was as follows (in thousands):
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
​ |  | 2020 |  | 2019 |  | 2018
Research and development | ​ | $ | 4,121 | ​ | $ | 4,350 | ​ | $ | 3,424
General and administrative | ​ |  | 3,778 | ​ |  | 4,003 | ​ |  | 3,495
Total stock-based compensation expense | ​ | $ | 7,899 | ​ | $ | 8,353 | ​ | $ | 6,919
Options to purchase common stock |
Black-Scholes option-pricing model assumptions | ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
​ | ​ | 2020 | ​ | 2019 | ​ | 2018
Expected term (in years) |  | 5.27 - 6.08 |  | 5.00 - 6.08 |  | 5.49 - 6.08
Expected volatility |  | 72.1% - 87.5% | ​ | 61.0% - 64.8% | ​ | 62.0% - 66.5%
Risk-free interest rate |  | 0.23% - 1.44% | ​ | 1.42% - 2.58% | ​ | 2.42% - 3.03%
Dividend yield |  | — |  | — |  | —
Restricted stock units |
Schedule of activity under equity incentive plans | ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Weighted
​ | ​ | ​ | ​ | Average
​ | ​ | Number of | ​ | Grant Date
​ |  | Shares |  | Fair Value
​ | ​ | ​ | ​ | ​ | ​
Unvested at December 31, 2019 | ​ | 278,482 | ​ | $ | 10.08
Granted |  | 142,000 |  | ​ | 7.80
Vested |  | (131,147) | ​ |  | 9.52
Forfeited | ​ | (44,790) | ​ | ​ | 8.67
Unvested at December 31, 2020 | ​ | 244,545 | ​ | $ | 9.31
ESPP rights |
Black-Scholes option-pricing model assumptions | ​
​ ​ ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | Year Ended December 31,
​ | ​ | 2020 |  | 2019 |  | 2018
Expected term (in years) | ​ | 0.50 | ​ | 0.50 | ​ | 0.50
Expected volatility | ​ | 89.1% - 120.4% | ​ | 58.9% - 65.3% | ​ | 49.0% - 63.4%
Risk-free interest rate | ​ | 0.12% - 0.43% | ​ | 1.89% - 2.32% | ​ | 1.89% - 2.32%
Dividend yield | ​ | — | ​ | — | ​ | —

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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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- Definition Tabular disclosure of activity for award under share-based payment arrangement. Includes, but is not limited to, outstanding award at beginning and end of year, granted, exercised, forfeited, 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 (e) -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 (c)(2) -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 (d) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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