CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
OPERATING ACTIVITIES:
Net loss	$ (72,148)	$ (25,991)	$ (21,785)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	882	684	582
Share-based compensation expense	6,764	1,982	1,259
Loss from equity-method investment	263	25	302
Other non-cash expenses	162	873	38
Changes in operating assets and liabilities:
Accounts receivable	54	2,302	(4,001)
Prepaid expenses and other assets	(2,011)	(120)	421
Accounts payable	4,812	3,155	578
Accrued liabilities	11,320	1,675	1,687
Deferred revenue	7,041	8,970	159
Net cash used in operating activities	(42,861)	(6,445)	(20,760)
INVESTING ACTIVITIES:
Acquisition of property and equipment	(1,742)	(818)	(1,478)
Purchases of short-term investments			(6,594)
Proceeds from maturities of short-term investments			30,206
Net cash (used in) provided by investing activities	(1,742)	(818)	22,134
FINANCING ACTIVITIES:
Proceeds from long-term debt, net of lender fees		4,945	9,872
Proceeds from exercise of stock options	2,726	139	332
Proceeds from issuance of common stock, net of issuance costs	423,819	21,278
Net cash provided by financing activities	436,145	41,907	10,204
NET INCREASE IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	391,542	34,644	11,578
Cash, cash equivalents and restricted cash at beginning of year	71,460	36,816	25,238
Cash, cash equivalents and restricted cash at end of year	463,002	71,460	36,816
Supplemental disclosure of cash flow information:
Cash paid for interest	751	691	146
Non-cash investing activities
Right-of-use asset obtained in exchange for lease liabilities	1,360	5,868
Sale of intangible assets for equity-method investment			590
Purchase of property and equipment in accounts payable	169	240	30
Release of repurchase liability for restricted shares			$ 39
Ultragenyx [Member]
FINANCING ACTIVITIES:
Proceeds from exercise of stock option by Ultragenyx and issuance of common stock and option, net of issuance costs, to Ultragenyx	$ 9,600	$ 15,545