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Stock-Based Compensation | 3 Months Ended
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Mar. 31, 2021
Stock-Based Compensation |
Stock-Based Compensation | 8. Stock-Based Compensation ​ The following table summarizes the stock option and award activity for the three months ended March 31, 2021: ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | Options Outstanding
​ | ​ | ​ | ​ | ​ | ​ | Weighted- | ​ | Weighted- | ​ | ​ | ​
​ | ​ | Options and | ​ | ​ | ​ | Average | ​ | Average | ​ | Aggregate
​ | ​ | Awards | ​ | ​ | ​ | Exercise | ​ | Remaining | ​ | Intrinsic
​ | ​ | Available for | ​ | Number of | ​ | Price Per | ​ | Contractual | ​ | Value
​ | ​ | Grant |  | Options |  | Share |  | Life (years) |  | (In thousands)
Balance—December 31, 2020 |  | 179,219 |  | 2,272,905 | ​ | $ | 5.58 |  | 7.9 | ​ | $ | 1,640
Authorized |  | 570,946 | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
RSUs granted | ​ | (236,605) | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
RSUs cancelled/forfeited | ​ | 31,626 | ​ | — | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Options granted | ​ | (39,500) | ​ | 39,500 | ​ | $ | 5.52 | ​ | ​ | ​ | ​ | ​
Options exercised |  | — | ​ | (54,077) | ​ | $ | 2.66 | ​ | ​ | ​ | $ | 194
Options cancelled/forfeited |  | 134,857 | ​ | (136,432) | ​ | $ | 3.96 | ​ | ​ | ​ | ​ | ​
Balance—March 31, 2021 |  | 640,543 |  | 2,121,896 | ​ | $ | 5.75 |  | 6.1 | ​ | $ | 2,359
Options exercisable—March 31, 2021 |  | ​ |  | 1,288,863 | ​ | $ | 6.81 | ​ | 4.5 | ​ | $ | 586
​ The total grant date fair value of options vested was $312,000 and $799,000 during the three months ended March 31, 2021 and 2020, respectively. ​ The weighted-average grant date fair value of employee options granted was $3.53 and $1.52 per share during the three months ended March 31, 2021 and 2020, respectively. ​ 2016 Employee Stock Purchase Plan ​ In January 2021, there was an increase of 190,315 shares reserved for issuance under the Company’s Employee Stock Purchase Plan (ESPP) pursuant to the terms of the ESPP. The Company had 717,181 shares available for future issuance under the Company’s ESPP as of March 31, 2021. Employees did not purchase any shares during the three months ended March 31, 2021 and 2020. ​ Restricted Stock Units ​ The following table summarizes restricted stock units (RSUs) activity for the three months ended March 31, 2021: ​
​ | ​ | ​ | ​ | ​ | ​
​ | ​ | RSUs Outstanding
​ | ​ | ​ |  | Weighted-
​ | ​ | ​ |  | Average
​ | ​ | Number of |  | Grant Date
​ | ​ | Restricted Stock |  | Fair Value Per
​ |  | Units |  | Share
Balance—December 31, 2020 | ​ | 416,742 |  | $ | 4.89
Granted |  | 236,605 |  | ​ | 5.71
Vested | ​ | (136,709) |  | ​ | 5.16
Cancelled/forfeited | ​ | (31,626) |  | ​ | 4.12
Balance—March 31, 2021 |  | 485,012 |  | $ | 5.26
​ The fair value of RSUs is determined on the date of grant based on the market price of the Company’s common stock on that date. As of March 31, 2021, there was $1.7 million of unrecognized stock-based compensation expense related to RSUs to be recognized over a weighted-average period of 1.5 years. ​ Stock-based Compensation Expense ​ As of March 31, 2021, there was $2.0 million of total unrecognized stock-based compensation expense related to unvested options which is expected to be recognized over a weighted-average period of 2.7 years. Compensation cost capitalized within inventory at March 31, 2021 and at December 31, 2020 was not material.

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