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Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Share-based Payment Arrangement [Abstract]

Stock-Based Compensation Expense

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

Assumptions Used for Estimating Fair Value of Employee Stock Options

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%

Weighted-Average Assumptions Used for Estimating Fair Value of ESPP Purchase Rights

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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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.

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Tabular disclosure of the significant assumptions used during the year to estimate the fair value of employee stock purchase plans, including, but not limited to: (a) expected term, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions.

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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.

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