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EQUITY INCENTIVE PLANS - Summary of Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 3 Months Ended 12 Months Ended
Mar. 31, 2022
Dec. 31, 2021
Number of Shares    
Outstanding, beginning balance (in shares) 8,296,091  
Options granted (in shares) 1,804,700  
Options exercised (in shares) 0  
Options canceled (in shares) (1,096,423)  
Outstanding, ending balance (in shares) 9,004,368 8,296,091
Vested and expected to vest (in shares) 9,004,368  
Exercisable (in shares) 4,609,717  
Weighted Average Exercise Price    
Outstanding, beginning balance (in dollars per share) $ 3.21  
Options granted (in dollars per share) 0.83  
Options exercised (in dollars per share) 0  
Options canceled (in dollars per share) 3.12  
Outstanding, ending balance (in dollars per share) 2.74 $ 3.21
Vested and expected to vest (in dollars per share) 2.74  
Exercisable (in dollars per share) $ 3.75  
Weighted Average Remaining Contractual Life in Years    
Outstanding 7 years 6 months 10 days 6 years 3 months 21 days
Vested and expected to vest 7 years 6 months 10 days  
Exercisable 5 years 10 months 9 days  
Total Aggregate Intrinsic Value (in thousands)    
Outstanding $ 4 $ 0
Vested and expected to vest 4  
Exercisable $ 0