4. FAIR VALUE OF FINANCIAL INSTRUMENTS (Details) - USD ($) $ in Thousands	Aug. 31, 2018	May 31, 2018
Money market funds	$ 13,277	$ 7,733
U.S. Treasury securities	0	5,983
Certificate of deposit	80	80
Assets	13,357	13,796
Level 1
Money market funds	13,277	7,733
U.S. Treasury securities	0	5,983
Certificate of deposit	0	0
Assets	13,277	13,716
Level 2
Money market funds	0	0
U.S. Treasury securities	0	0
Certificate of deposit	80	80
Assets	80	80
Level 3
Money market funds	0	0
U.S. Treasury securities	0	0
Certificate of deposit	0	0
Assets	$ 0	$ 0