INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
U.S. Federal and State
U.S. federal and state deferred tax assets—long term:
Accrued payroll	$ 180	$ 261
Accrued expenses	53	77
Fixed assets	17	0
Charitable contributions	14	0
Intangibles	431	390
Research & development credit	3,830	3,651
Net operating loss	35,279	27,568
Stock compensation	519	672
Lease liabilities	700	750
New jobs credit	8	8
Total deferred tax assets	41,031	33,377
U.S. federal and state deferred tax liabilities—long term:
Right of use assets	(621)	(701)
Total deferred tax liabilities	(621)	(701)
Total long-term assets	40,410	32,676
Less: Valuation allowance	(40,410)	(32,676)
Net deferred tax assets	0	0
Foreign
U.S. federal and state deferred tax assets—long term:
Net operating loss	6	8
Total deferred tax assets	6	8
U.S. federal and state deferred tax liabilities—long term:
Less: Valuation allowance	(6)	(8)
Net deferred tax assets	$ 0	$ 0