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STOCK-BASED COMPENSATION - Stock-based compensation expenses (Details) - USD ($)
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Total stock compensation expenses $ 1,344,592 $ 1,525,976
Research and development    
Total stock compensation expenses 781,000 687,000
General and administrative    
Total stock compensation expenses $ 564,000 $ 839,000