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STOCK-BASED COMPENSATION - Summary of Restricted Stock Units Activity (Details) - Restricted stock units
9 Months Ended
Sep. 30, 2021
$ / shares
shares
Number of Shares  
Unvested, Balance at beginning of period (in shares) | shares 843,095
Granted (in shares) | shares 3,379,467
Vested (in shares) | shares (38,533)
Forfeited (in shares) | shares (248,800)
Unvested, Balance at end of period (in shares) | shares 3,935,229
Weighted-Average Grant Date Fair Value  
Unvested, Balance at beginning of period (in dollars per share) | $ / shares $ 2.25
Granted (in dollars per share) | $ / shares 5.12
Vested (in dollars per share) | $ / shares 0.56
Forfeited (in dollars per share) | $ / shares 4.37
Unvested, Balance at end of period (in dollars per share) | $ / shares $ 4.60