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Stock-Based Compensation | 12 Months Ended
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Dec. 31, 2020
Share-based Payment Arrangement [Abstract] |
Stock-Based Compensation | STOCK-BASED COMPENSATION The following table shows total stock-based compensation expense recognized in the accompanying Consolidated Statements of Operations (in thousands):
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Research and development | $ | 13,523 |  |  | $ | 10,135 |  |  | $ | 8,249 |
General and administrative | 12,185 |  |  | 9,195 |  |  | 6,428 |
Total stock-based compensation expense | $ | 25,708 |  |  | $ | 19,330 |  |  | $ | 14,677 |
As of December 31, 2020, total stock-based compensation expense to be recognized in future periods related to unvested stock options was $35.5 million, which is expected to be expensed over a weighted-average period of 2.72 years. As of December 31, 2020, total stock-based compensation expense to be recognized in future periods related to unvested RSUs was $17.1 million, which is expected to be expensed over a weighted-average period of 2.07 years. There was no capitalized stock-based employee compensation expense as of December 31, 2020, 2019 or 2018. Valuation Assumptions Employee stock-based compensation expense was determined using the Black-Scholes option valuation model for stock options and employee share purchases under the ESPP. Option valuation models require the input of subjective assumptions and these assumptions can vary over time. The fair value of RSUs was based on the closing price of the underlying common stock on the date of grant. The Company bases its determination of expected volatility through its assessment of the historical volatility of its common stock. The Company relied on its historical exercise and post-vested termination activity for estimating its expected term for use in determining the fair value of these options. The weighted-average estimated fair value per share of options granted during 2020, 2019 and 2018 was $5.25, $6.37, and $11.39, respectively, based upon the assumptions used in the Black-Scholes valuation model. The assumptions used for estimating the fair value of the employee stock options were as follows:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Risk-free interest rate | 0.34-0.61% |  | 1.68-2.25% |  | 2.53-2.96%
Expected term (in years) | 5.51-5.57 |  | 5.50-5.62 |  | 5.59-5.61
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 77.61-80.32% |  | 76.46-78.39% |  | 72.33-75.49%
Employees purchased 274,382, 249,364 and 328,710 shares of common stock through the ESPP at an average exercise price of $7.34, $8.53, and $4.51 per share during 2020, 2019 and 2018, respectively. The weighted-average estimated fair values of shares purchased under the Company’s ESPP during 2020, 2019 and 2018 were $8.02, $4.70 and $7.07, respectively, based upon the assumptions used in the Black-Scholes valuation model. The assumptions used for estimating the fair value of the ESPP purchase rights are as follows:
| Year Ended December 31,
| 2020 |  | 2019 |  | 2018
Risk-free interest rate | 1.53-2.80% |  | 1.53-2.42% |  | 2.16-2.80%
Expected term (in years) | 0.5-2.0 |  | 0.5-2.0 |  | 0.5-2.0
Expected dividend yield of stock | — |  |  | — |  |  | — |
Expected volatility | 51.02-91.96% |  | 51.02-91.96% |  | 73.21-83.25%

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- Definition The entire disclosure for share-based payment arrangement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -URI http://asc.fasb.org/topic&trid=2228938 + Details
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