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Stock-Based Compensation (Tables)

3 Months Ended

Mar. 31, 2020

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Summary of Stock Option Activity Under Plan

The following summarizes option activity under the 2018 Plan:

			Outstanding Options
			Number of Options						Weighted- Average Exercise Price						Weighted- Average Remaining Contract Term						Aggregate intrinsic value
															(in years)						(in thousands)
Balance, December 31, 2019			9,190,522						$	14.51					8.82						$	110,490
Granted			2,675,672						18.38
Exercised			(105,895)						2.87
Forfeited			(132,910)						9.58
Balance, March 31, 2020			11,627,389						$	15.56					8.87						$	77,096
Exercisable, March 31, 2020			6,716,452						$	13.66					8.90						$	51,873
Vested and expected to vest, March 31, 2020			11,627,389						$	15.56					8.87						$	77,096

Schedule Stock Option Valuation Assumptions

The fair value of employee, consultant and director stock option awards was estimated at the date of grant using a Black-Scholes option-pricing model with the following assumptions:

			Three Months Ended March 31,
			2020						2019
Expected term in years			6.07 - 6.09						5.99 - 6.25
Expected volatility			71.42%						74.84%
Expected risk-free interest rate			0.80% - 1.65%						2.45% - 2.62%
Expected dividend			0%						0%

Schedule of Restricted Stock Units Activity Under Plan

The following summarizes restricted stock unit activity under the 2018 Plan:

			Outstanding Restricted Stock Units
			Restricted Stock Units						Weighted- Average Fair Value at Date of Grant per Share						Weighted Average Remaining Vesting Life						Aggregate Intrinsic Value
															(in years)						(in thousands)
Unvested December 31, 2019			1,941,155						$	27.45					1.98						$50,431
Granted			885,267						18.55						2.44
Vested			(270,751)						27.32
Forfeited			(21,000)						28.19
Unvested March 31, 2020			2,534,671						$	24.35					2.17						$49,274
Vested and expected to vest, March 31, 2020			2,534,671						$	24.35					2.17						$49,274

Schedule of Stock-Based Compensation Expense

Total stock-based compensation related to stock options, restricted stock units, employee stock purchase plan and vesting of the founders’ common stock was as follows (in thousands):

			Three Months Ended March 31
			2020						2019
Research and development			$	6,657					$	2,732
General and administrative			7,558						5,135
Total stock-based compensation			$	14,215					$	7,867

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Tabular disclosure of the allocation of equity-based compensation costs to a given line item on the balance sheet and income statement for the period. This may include the reporting line for the costs and the amount capitalized and expensed.

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Tabular disclosure of the changes in outstanding nonvested restricted stock units.

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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.

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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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