CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities
Net loss	$ (29,305)	$ (12,140)
Adjustments to reconcile net loss to net cash used in operating activities:
Stock-based compensation	5,596	1,081
Depreciation and amortization	570	536
Accretion on marketable securities	(1,288)	0
Amortization of debt issuance costs	194	194
Change in fair value of redeemable convertible preferred stock warrants	0	681
Loss on sale and disposal of property and equipment	152	95
Changes in operating assets and liabilities:
Accounts receivable	(1,383)	354
Inventory	(1,575)	(458)
Prepaid expenses and other assets	394	495
Accounts payable	526	(884)
Accrued liabilities and other	3,525	1,844
Net cash used in operating activities	(22,594)	(8,202)
Cash flows from investing activities
Maturities of marketable securities	117,100	0
Purchases of marketable securities	(107,545)	0
Purchases of property and equipment	(1,639)	(777)
Net cash provided by (used in) investing activities	7,916	(777)
Cash flows from financing activities
Proceeds from the exercise of stock options	1,178	915
Proceeds from issuance of common stock under employee stock purchase plan	1,263	0
Repurchase of unvested early exercised stock options	(38)	0
Payments of public offering costs	(167)	(288)
Net cash provided by financing activities	2,236	627
Effect of exchange rate changes on cash and cash equivalents	(64)	(52)
Net decrease in cash and cash equivalents	(12,506)	(8,404)
Cash and cash equivalents at
Beginning of period	25,120	22,408
End of period	12,614	14,004
Supplemental disclosure of non-cash information
Vesting of early exercised stock options	161	64
Purchases of property and equipment included in accounts payable and accrued liabilities	31	35
Public offering costs included in accounts payable	$ 0	$ 865