STOCK BASED COMPENSATION - Stock-based compensation expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Stock-based compensation
Total stock-based compensation expense	$ 2,231	$ 2,702	$ 4,308	$ 5,683
Research and development
Stock-based compensation
Total stock-based compensation expense	788	533	1,644	1,332
General and administrative
Stock-based compensation
Total stock-based compensation expense	$ 1,443	$ 2,169	$ 2,664	$ 4,351