CONDENSED CONSOLIDATED BALANCE SHEETS (Unaudited) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 71,133	$ 34,080
Marketable securities	32,275	81,363
Other receivables	2,073	376
Tax incentive receivable, current	921	922
Prepaid expenses and other current assets	6,288	7,478
Total current assets	112,690	124,219
Property and equipment, net	2,689	2,893
Tax incentive receivable	474	233
Operating lease right of use assets	4,187
Other assets	2,449	1,661
Total assets	122,489	129,006
Current liabilities:
Accounts payable	1,039	3,603
Accrued expenses and other current liabilities	6,416	8,263
Derivative liabilities	223	223
Deferred rent		229
Current operating lease liabilities	571
Total current liabilities	8,249	12,318
Deferred rent, net of current portion		833
Non-current operating lease liabilities	4,278
Other long-term liabilities	293
Total liabilities	12,820	13,151
Commitments and contingencies (Note 9)
Stockholders' equity:
Preferred stock, $0.001 par value; 10,000,000 shares authorized, 2,720 shares issued and outstanding as of June 30, 2019 and 3,220 shares issued and outstanding as of December 31, 2018
Common stock, $0.001 par value; 60,000,000 shares authorized as of June 30, 2019 and December 31, 2018;18,650,375 shares issued and outstanding as of June 30, 2019 and 17,205,962 shares issued and outstanding as of December 31, 2018	19	17
Additional paid-in capital	266,013	254,013
Accumulated deficit	(156,724)	(138,502)
Accumulated other comprehensive gain (loss)	6	(28)
Total Spero Therapeutics, Inc. stockholders' equity	109,314	115,500
Non-controlling interests	355	355
Total stockholders' equity	109,669	115,855
Total liabilities and stockholders' equity	$ 122,489	$ 129,006