Property and Equipment - Summary of Property and Equipment (Details) - USD ($) $ in Thousands	Oct. 31, 2017	Apr. 30, 2017
Property, Plant and Equipment [Abstract]
Furniture and fixtures	$ 73	$ 74
Computer equipment and software	956	872
Laboratory equipment	2,232	918
Assets in progress	19	472
Leasehold improvements	0	2
Total property and equipment	3,280	2,338
Less: Accumulated depreciation	(1,254)	(1,122)
Property and equipment, net	$ 2,026	$ 1,216