Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2021	Apr. 30, 2020	Apr. 30, 2019
Cash flows from operating activities:
Net loss	$ (46,244)	$ (29,116)	$ (20,816)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	537	512	378
Stock-based compensation expense	7,118	4,448	2,966
Realized gain from sale of marketable securities	(153)	(300)	(23)
Non-cash operating lease expense	114	13
Amortization of premium on available for sale securities	685	193
Foreign currency exchange (gain) loss	(574)	74	(80)
Changes in operating assets and liabilities:
Research and development tax credit receivable	7,457	(5,781)	(4,883)
Prepaid expenses and other current assets	(222)	(1,112)	(1,979)
Other assets	(22)	(5)
Accounts payable	150	(1,004)	1,534
Accrued expenses	983	(48)	2,665
Deferred revenue		(12,690)	(16,127)
Net cash used in operating activities	(30,171)	(44,816)	(36,365)
Cash flows from investing activities:
Purchases of available for sale securities	(201,210)	(49,797)	(79,889)
Sales and maturities of available for sale securities	53,638	66,770	11,548
Acquisition of property and equipment	(82)	(220)	(1,081)
Net cash provided by (used in) investing activities	(147,654)	16,753	(69,422)
Cash flows from financing activities:
Issuance of common stock, net of offering expenses	210,582	11,422	87,910
Issuance of common stock from equity incentive plans	1,535	216	242
Finance lease principal payments		(54)	(209)
Net cash provided by financing activities	212,117	11,584	87,943
Effect of exchange rate changes on cash and cash equivalents	511	262	(1,205)
Net increase (decrease) in cash and cash equivalents	34,803	(16,217)	(19,049)
Cash and cash equivalents, beginning year	15,789	32,006	51,055
Cash and cash equivalents, end of year	50,592	15,789	$ 32,006
Supplemental disclosures of cash flow information:
Right of use assets obtained in exchange for operating lease liabilities	$ 4,784	$ 309