|
Income taxes (Details 3) (USD $)
|
12 Months Ended |
|
Dec. 31, 2014
|
Dec. 31, 2013
|
Dec. 31, 2012
|
| Reconciliation of income taxes computed using the U.S. federal statutory rate |
|
|
|
| Income tax benefit using U.S. federal statutory rate (as a percent) |
34.00% |
34.00% |
|
| State tax benefit, net of federal benefit (as a percent) |
4.73% |
4.69% |
|
| Research and development tax credits (as a percent) |
1.51% |
1.67% |
|
| Permanent items (as a percent) |
(6.25%) |
(4.76%) |
|
| Change in the valuation allowance (as a percent) |
(34.34%) |
(35.20%) |
|
| Other (as a percent) |
0.35% |
(0.40%) |
|
| Deferred tax assets: |
|
|
|
| Net operating loss carryforwards |
$ 42,352,000 |
$ 25,782,000 |
|
| Capitalized research and development |
2,154,000 |
1,839,000 |
|
| Research and development credits |
2,880,000 |
1,856,000 |
|
| Stock-based compensation |
726,000 |
511,000 |
|
| Other |
199,000 |
(4,000) |
|
| Gross deferred tax assets |
48,311,000 |
29,984,000 |
|
| Valuation allowance |
(48,311,000) |
(29,984,000) |
|
| Increase in valuation allowance |
18,300,000 |
14,500,000 |
10,300,000 |
| Adjustment for unrecognized income tax benefits |
0 |
|
|
| Unrecognized tax benefits |
0 |
|
|
| Interest and penalties accrued related to unrecognized tax benefits |
0 |
|
|
| Uncertain tax positions |
$ 0 |
|
|