7. INCOME TAXES (Details 2)	12 Months Ended
May 31, 2017	May 31, 2016	May 31, 2015
Reconciliation of Federal Statutory Rate to Effective Rate
U.S. federal statutory tax rate	34.00%	34.00%	34.00%
State taxes, net of federal tax effect	(0.10%)	0.00%	(0.20%)
Foreign rate differential	0.10%	0.20%	1.40%
Stock-based compensation	(2.80%)	(3.80%)	(2.20%)
Research and development credit	3.10%	2.10%	1.10%
Change in valuation allowance	(33.80%)	(32.50%)	(34.40%)
Other	(0.90%)	(0.20%)	(0.20%)
Effective tax rate	(0.40%)	(0.20%)	(0.50%)

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to statutory income tax expense (benefit) outside of the country of domicile.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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