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SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - STOCK BASED COMPENSATION (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Stock-Based Compensation Expense        
Stock-based compensation expense included in operating expenses $ 2,008 $ 1,768 $ 5,775 $ 5,043
Research and development        
Stock-Based Compensation Expense        
Stock-based compensation expense included in operating expenses 924 623 2,655 1,703
General and administrative        
Stock-Based Compensation Expense        
Stock-based compensation expense included in operating expenses $ 1,084 $ 1,145 $ 3,120 $ 3,340