CONSOLIDATED STATEMENTS OF CASH FLOWS	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Cash flows from operating activities:
Net loss	$ (56,671,000)	$ (27,319,000)	$ (21,873,000)
Adjustments to reconcile net loss to cash used in operating activities:
Depreciation and amortization	2,360,000	741,000	825,000
Stock-based compensation expense	8,178,000	4,644,000	4,829,000
Restricted stock-based compensation expense	688,000	1,272,000	3,432,000
Non-cash earnings on investments	(626,000)	0	0
Net gain on write-off of licensed technology	0	(127,000)	0
Change in operating assets and liabilities:
Receivables	26,000	17,000	(9,000)
Prepaid expenses and other current assets	(1,067,000)	(2,580,000)	160,000
Other assets	40,000	(11,000)	(4,000)
Accounts payable and accrued expenses	7,665,000	(1,301,000)	2,819,000
Contingent consideration milestone	0	0	(2,591,000)
Deferred revenue	296,000	2,009,000	(602,000)
Net cash used in operating activities	(39,111,000)	(22,655,000)	(13,014,000)
Cash flows from investing activities:
Capital expenditures	(9,243,000)	(860,000)	(519,000)
Acquisition of licensed technology	(10,251,000)	0	0
Purchases of short-term investments	(136,092,000)	0	0
Proceeds from maturities of short-term investments	70,500,000	0	0
Net cash used in investing activities	(85,086,000)	(860,000)	(519,000)
Cash flows from financing activities:
Proceeds from exercise of stock options	2,244,000	344,000	350,000
Proceeds from $2.85 restricted common stock issuance	0	0	150,000
Net cash provided by financing activities	5,477,000	92,403,000	42,537,000
Net (decrease)/increase in cash, cash equivalents and restricted cash	(118,720,000)	68,888,000	29,004,000
Cash, cash equivalents and restricted cash at beginning of year	138,030,000	69,142,000	40,138,000
Cash, cash equivalents and restricted cash at end of year	19,310,000	138,030,000	69,142,000
Supplemental cash flow information:
Cash and cash equivalents	18,750,000	137,750,000	69,142,000
Restricted cash	560,000	280,000	0
Total cash, cash equivalents and restricted cash	19,310,000	138,030,000	69,142,000
Payable for acquistion of licensed technology	30,000,000	0	0
Write off of licensed asset and corresponding liability	0	4,000,000	0
Shares issued to EB Research Partnership and Epidermolysis Bullosa Medical Research Foundation for licenses	0	0	2,452,000
Cash paid for interest	11,000	8,000	6,000
Private Placement One [Member]
Cash flows from financing activities:
Proceeds From Issuance Of Common Stock	0	86,174,000	0
Private Placement Two [Member]
Cash flows from financing activities:
Proceeds From Issuance Of Common Stock	0	0	41,068,000
Private Placement Three [Member]
Cash flows from financing activities:
Proceeds From Issuance Of Common Stock	0	0	969,000
Dollar Five Warrant [Member]
Cash flows from financing activities:
Proceeds from Warrant Exercises	3,233,000	885,000	0
Dollar Eight Warrant [Member]
Cash flows from financing activities:
Proceeds from Warrant Exercises	$ 0	$ 5,000,000	$ 0