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STOCK-BASED COMPENSATION (Tables)
 12 Months Ended
Dec. 31, 2019
Share-based Payment Arrangement [Abstract]  
Schedule of Stock Option Activity
The following table summarizes stock option activity under the plans for the year ended December 31, 2019.
 SharesWeighted-
Average
Exercise
Price
Weighted-
Average
Remaining
Contractual
Term
(years)
Aggregate
Intrinsic
Value
(thousands)
Balance at December 31, 20184,577,515  $5.99  8.0$84,290  
Granted3,720,091  29.71
Exercised(1,108,700) 1.77
Forfeited or canceled(390,923) 28.29
Balance at December 31, 20196,797,983  $18.38  8.4$133,892  
Vested and expected to vest at December 31, 20196,471,442  $17.96  8.3$129,998  
Schedule of Restricted Stock Activity
The following table summarizes RSU activity under the 2018 Plan for the year ended December 31, 2019.
 SharesWeighted-
Average
Grant Date
Fair Value 		 
Unvested balance at December 31, 201821,792  $19.00  
Granted11,274  36.74
Vested(21,792) 19.00
Forfeited—  
Unvested balance at December 31, 201911,274  $36.74  
Schedule of Stock Options, Valuation Assumptions
The fair value of the stock options granted to employees for the years ended December 31, 2019, 2018 and 2017 was calculated with the following assumptions.
 Years Ended December 31,
 201920182017
Risk-free interest rate2.1 %2.7 %1.7 %
Expected volatility73.6 %64.6 %
72.7% - 78.5%
Expected term (in years)5.95.8
6.2 - 6.3
Expected dividends— %— %— %
The fair value of the stock options granted to nonemployees for the years ended December 31, 2019, 2018 and 2017 was calculated with the following assumptions.
 Years Ended December 31,
 201920182017
Risk-free interest rate2.5 %2.7 %
1.7% - 2.2%
Expected volatility76.4 %68.4 %
70.8% - 73.1%
Expected term (in years)6.410.0
6.5 - 8.9
Expected dividends— %— %— %
Schedule of Weighted Average Fair Value and Intrinsic Value
The following table summarizes the weighted-average fair value per share of stock options granted and the total intrinsic value of stock options exercised for the years ended December 31, 2019, 2018 and 2017.
Years Ended December 31,
(in thousands, except per share amounts)201920182017
Stock options granted - weighted-average grant date fair value per share
$19.28  $10.39  $1.67  
Stock options exercised - intrinsic value34,416  1,935  30  
Schedule of Stock-based Compensation Expense
The following table presents stock-based compensation expense as reported in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2019, 2018 and 2017.
 Years Ended December 31,
(in thousands)201920182017
Research and development$13,547  $2,643  $379  
General and administrative11,621  2,509  497  
Total$25,168  $5,152  $876  
The following table presents stock-based compensation expense by award type as reported in the Company’s statements of operations and comprehensive loss for the years ended December 31, 2019, 2018 and 2017.
 Years Ended December 31,
(in thousands)201920182017
Stock options$24,271  $4,806  $876  
RSUs447  202  —  
ESPP450  144  —  
Total$25,168  $5,152  $876