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Stock-Based Compensation - Stock based compensation expense (Details) - USD ($)
$ in Thousands
3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Stock based compensation expense    
Total stock-based compensation expense $ 2,508 $ 1,008
Research and development    
Stock based compensation expense    
Total stock-based compensation expense 958 452
General and administrative    
Stock based compensation expense    
Total stock-based compensation expense $ 1,550 $ 556