Summary of significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2018

New Accounting Pronouncements or Change in Accounting Principle [Line Items]

Cash, cash equivalents and marketable securities

Cash, cash equivalents, and marketable securities as of December 31, 2018 and 2017 consist of the following:

	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
	(in thousands)
As of December 31, 2018
Money market funds included in cash and cash equivalents	$	46,173	$	—	$	—	$	46,173
Marketable securities:
U.S. Treasury notes		108,951		1		5		108,947
Equity securities		1,220		—		592		628
Total marketable securities	$	110,171	$	1	$	597	$	109,575
Total money market funds and marketable securities	$	156,344	$	1	$	597	$	155,748
As of December 31, 2017
Money market funds included in cash and cash equivalents	$	30,469	$	—	$	—	$	30,469
Marketable securities:
U.S. Treasury notes		137,560		—		38		137,522
Equity securities		1,220		—		120		1,100
Total marketable securities	$	138,780	$	—	$	158	$	138,622
Total money market funds and marketable securities	$	169,249	$	—	$	158	$	169,091

Reconciliation of cash, cash equivalents, and restricted cash

	As of December 31,
	2018		2017		2016
	(in thousands)
Cash and cash equivalents	$	46,859	$	31,530	$	36,641
Restricted cash included in deposits and other noncurrent assets		735		735		735
Total cash, cash equivalents, and restricted cash	$	47,594	$	32,265	$	37,376

Outstanding potentially dilutive securities excluded in the calculation of diluted net loss per share

The following table sets forth the outstanding potentially dilutive securities that have been excluded in the calculation of diluted net loss per share because to do so would be anti‑dilutive:

	As of December 31,
	2018	2017	2016
Unvested restricted common stock	235,294	557,979	1,167,984
Outstanding stock options	4,225,152	3,143,566	1,871,237
Total	4,460,446	3,701,545	3,039,221

ASU 2014-09

New Accounting Pronouncements or Change in Accounting Principle [Line Items]

Schedule of impact of adoption of financial statement line items shows comparative balances under ASC 606 and ASC 605

Condensed Consolidated Statements of Operations and Comprehensive Loss

	Year ended December 31, 2018
	Under ASC 606		Under ASC 605		Effect of change
	(in thousands, except per share data)
Collaboration revenue	$	7,619	$	11,095	$	(3,476)
Loss before income taxes		(88,468)		(84,992)		(3,476)
Net loss		(88,288)		(84,812)		(3,476)
Net loss per share, basic and diluted		(2.75)		(2.64)		(0.11)

Condensed Consolidated Balance Sheets

	As of December 31, 2018
	Under ASC 606		Under ASC 605		Effect of change
	(in thousands)
Deferred revenue, current	$	20,847	$	19,111	$	1,736
Deferred revenue, non-current		92,199		70,529		21,670
Accumulated deficit		(269,051)		(245,645)		(23,406)

Condensed Consolidated Statements of Cash Flows

	Year ended December 31, 2018
	Under ASC 606		Under ASC 605		Effect of change
	(in thousands)
Net loss	$	(88,288)	$	(84,812)	$	(3,476)
Adjustments to reconcile net loss to net cash used in operating activities:
Deferred revenue		61,380		57,904		3,476