CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 49,395	$ 34,080
Marketable securities	56,997	81,363
Other receivables	456	376
Tax incentive receivable, current	928	922
Prepaid expenses and other current assets	7,926	7,478
Total current assets	115,702	124,219
Property and equipment, net	2,864	2,893
Tax incentive receivable	294	233
Operating lease right of use assets	4,369
Other assets	1,421	1,661
Total assets	124,650	129,006
Current liabilities:
Accounts payable	557	3,603
Accrued expenses and other current liabilities	5,199	8,263
Derivative liabilities	223	223
Deferred rent		229
Current operating lease liabilities	639
Total current liabilities	6,618	12,318
Deferred rent, net of current portion		833
Non-current operating lease liabilities	4,391
Other long-term liabilities	312
Total liabilities	11,321	13,151
Commitments and contingencies (Note 9)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized, 3,220 shares issued and outstanding as of March 31, 2019 and December 31, 2018
Common stock, $0.001 par value; 60,000,000 shares authorized as of March 31, 2019 and December 31, 2018;17,335,066 shares issued and outstanding as of March 31, 2019 and 17,205,962 shares issued and outstanding as of December 31, 2018	17	17
Additional paid-in capital	256,530	254,013
Accumulated deficit	(143,574)	(138,502)
Accumulated other comprehensive gain (loss)	1	(28)
Total Spero Therapeutics, Inc. stockholders' equity	112,974	115,500
Non-controlling interests	355	355
Total stockholders' equity	113,329	115,855
Total liabilities and stockholders' equity	$ 124,650	$ 129,006