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STOCK-BASED COMPENSATION - weighted-average assumptions (Details)
3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
STOCK-BASED COMPENSATION        
Expected volatility 108.00% 109.00% 108.00% 109.00%
Expected term 5 years 5 years 5 years 5 years
Risk-free interest rate 2.21% 2.65% 2.24% 2.44%
Expected dividend yield 0.00% 0.00% 0.00% 0.00%