Fair Value Measurements

6 Months Ended

Jun. 30, 2018

Fair Value Disclosures [Abstract]

FAIR VALUE MEASUREMENTS

4. FAIR VALUE MEASUREMENTS

The Company’s financial instruments consist of cash and cash equivalents, short-term investments, restricted cash and accounts payable. The carrying amount of cash, restricted cash and accounts payable are each considered a reasonable estimate of fair value due to the short-term maturity.

Assets measured at fair value on a recurring basis were as follows:

Description	As of June 30, 2018		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents:
Money market mutual funds	$	35,781,822	$	35,781,822	$	—	$	—
Commercial paper		32,876,471		—		32,876,471		—
Corporate debt securities		5,643,984		—		5,643,984		—
U.S. Treasury securities		20,094,911		—		20,094,911		—
Total cash equivalents	$	94,397,188	$	35,781,822	$	58,615,366	$	—
Short-term investments:
Asset-backed securities	$	20,159,772	$	—	$	20,159,772	$	—
Commercial paper		53,019,143		—		53,019,143		—
Corporate debt securities		58,467,890		—		58,467,890		—
U.S. Treasury securities		28,900,745		—		28,900,745		—
Total short-term investments	$	160,547,550	$	—	$	160,547,550	$	—
Total financial assets	$	254,944,738	$	35,781,822	$	219,162,916	$	—
Description	As of December 31, 2017		Quoted Prices in Active Markets for Identical Assets (Level 1)		Significant Other Observable Inputs (Level 2)		Significant Unobservable Inputs (Level 3)
Cash equivalents:
Money market mutual funds	$	44,181,756	$	44,181,756	$	—	$	—
Total cash equivalents	$	44,181,756	$	44,181,756	$	—	$	—
Short-term investments:
Asset-backed securities	$	7,421,703	$	—	$	7,421,703	$	—
Commercial paper		34,882,298		—		34,882,298		—
Corporate debt securities		26,856,283		—		26,856,283		—
U.S. Treasury securities		8,923,320		—		8,923,320		—
Total short-term investments	$	78,083,604	$	—	$	78,083,604	$	—
Total financial assets	$	122,265,360	$	44,181,756	$	78,083,604	$	—

Short-term securities are valued using models or other valuation methodologies that use Level 2 inputs. These models are primarily industry-standard models that consider various assumptions, including time value, yield curve, volatility factors, default rates, current market and contractual prices for the underlying financial instruments, as well as other relevant economic measures. Substantially all of these assumptions are observable in the marketplace, can be derived from observable data or are supported by observable levels at which transactions are executed in the marketplace.

The convertible preferred stock tranche liability was stated at fair value and was measured using a Level 3 input because the fair value measurement was based, in part, on significant inputs not observed in the market.

The reconciliation of changes in the fair value of financial instruments based on Level 3 inputs for the six months ended June 30, 2017, consisted of:

Fair value as of January 1, 2017	$	4,247,000
Change in fair value of convertible preferred stock tranche    liability		876,000
Reduction in tranche liability due to preferred stock issuance		(5,123,000	)
Fair value as of June 30, 2017	$	—

There were no transfers between fair value measure levels during the three and six months ended June 30, 2018 and 2017.