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2. STOCK-BASED COMPENSATION - Options (Details 1) (Stock Options, USD $)
3 Months Ended 6 Months Ended
Nov. 30, 2013
Nov. 30, 2012
Nov. 30, 2013
Nov. 30, 2012
Stock Options
       
Expected term (in years) 4 years 5 years 4 years 5 years
Volatility 94.00% 93.00% 95.00% 91.00%
Expected dividend $ 0 $ 0 $ 0 $ 0
Risk-free interest rates 0.95% 0.64% 1.40% 0.72%
Estimated forfeiture rate 0.25% 0.25% 0.25% 0.25%
Weighted average grant date fair value $ 1.77 $ 0.57 $ 1.03 $ 0.79