Accrued expenses and other current liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued expenses and other current liabilities
Accrued employee compensation and benefits	$ 4,390	$ 2,515
Accrued external research and development expenses	4,428	2,055
Accrued professional fees	1,759	867
Current portion of operating lease liabilities	844	640
Other	208	67
Accrued expenses and other current liabilities	$ 11,629	$ 6,144