Income taxes (Tables)

12 Months Ended

Dec. 31, 2018

Schedule of effective income tax rate reconciliation

The following table presents a reconciliation of income tax expense (benefit) computed at the statutory federal income tax rate to the effective income tax rate as reflected in the consolidated financial statements:



	Year Ended December 31, 2018		Year Ended December 31, 2017		Year Ended December 31, 2016
Federal income tax expense at statutory rate	21.0	%	35.0	%	35.0	%
State income tax, net of federal benefit	6.0	%	5.0	%	5.0	%
Permanent differences	(0.1	)%	(0.7	)%	0.0	%
Stock-based compensation	(2.8	)%	(1.9	)%	(2.6	)%
Research credits	2.6	%	2.2	%	1.9	%
Other, net	(1.6	)%	(1.3	)%	(0.1	)%
Payroll tax credit election	(1.1	)%	(0.7	)%	0.0	%
Change in valuation allowance	(24.0	)%	(1.1	)%	(39.2	)%
Deferred rate change	0.0	%	(36.5	)%	0.0	%
Effective tax rate	0.0	%	0.0	%	0.0	%

Schedule of deferred tax assets and liabilities

The following table presents the significant components of the Company's deferred tax assets and liabilities:



	December 31, 2018			December 31, 2017
Deferred tax assets:
U.S. and state net operating loss carryforwards	$	29,902,019		$	24,744,841
Accruals and other temporary differences	336,229			202,301
Amortization	50,688			35,783
Stock-based compensation	1,220,546			1,591,131
Tax credit carryforward	2,277,428			1,964,189
Total deferred tax assets	33,786,910			28,538,245
Less valuation allowance	(33,786,910		)	(28,533,755		)
Deferred tax assets	—			4,490
Deferred tax liabilities:
Stock-based compensation	—			(4,490		)
Deferred tax liabilities	—			(4,490		)
Net deferred tax assets	$	—		$	—