Quarterly report pursuant to Section 13 or 15(d)

7. STOCKHOLDERS' EQUITY (Details)

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7. STOCKHOLDERS' EQUITY (Details)
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
STOCKHOLDERS' EQUITY    
Expected term, minimum 5 years 9 months 18 days 5 years 9 months 18 days
Expected term, maximum 6 years 3 months 18 days 6 years 3 months 18 days
Volatility, minimum 54.20% 54.00%
Volatility, maximum 62.30% 57.00%
Dividend yield 0.00% 0.00%
Risk-free interest rate, minimum 1.80% 2.40%
Risk-free interest rate, maximum 2.92% 2.59%