CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 40,480	$ 87,288
Marketable securities	90,745
Other receivables	332	1,011
Tax incentive receivable, current	944	1,932
Prepaid expenses and other current assets	1,222	1,828
Total current assets	133,723	92,059
Property and equipment, net	2,191	1,164
Deposits	206	206
Restricted cash		50
Total assets	136,120	93,479
Current liabilities:
Accounts payable	1,364	3,470
Accrued expenses and other current liabilities	8,564	4,321
Derivative liabilities	223	223
Deferred rent	127	143
Total current liabilities	10,278	8,157
Deferred rent, net of current portion	290	365
Total liabilities	10,568	8,522
Commitments and contingencies (Note 9)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized, 2,220 shares issued and outstanding as of September 30, 2018 and zero shares issued and outstanding as of December 31, 2017
Common stock, $0.001 par value; 60,000,000 shares authorized as of September 30, 2018 and December 31, 3017; 18,182,494 shares issued and outstanding as of September 30, 2018 and 14,369,182 shares issued and outstanding as of December 31, 2017	18	14
Additional paid-in capital	253,105	181,428
Accumulated deficit	(127,903)	(96,840)
Accumulated other comprehensive loss	(23)
Total Spero Therapeutics, Inc. stockholders' equity	125,197	84,602
Non-controlling interests	355	355
Total stockholders' equity	125,552	84,957
Total liabilities and stockholders' equity	$ 136,120	$ 93,479