Summary of significant accounting policies (Tables)

6 Months Ended

Jun. 30, 2012

Changes in the allowance for doubtful accounts

The following table presents the changes in the allowance for doubtful accounts:




	Years ended December 31,									Six months ended June 30,
(in thousands)	2009			2010			2011			2012
										(unaudited)
Allowance—beginning of period	$	(5	)	$	—		$	(10	)	$	—
Provisions for bad debts	(52		)	(10		)	—			—
Write-off, net of recoveries and other	57			—			10			—

Allowance—end of period	$	—		$	(10	)	$	—		$	—

Fair value hierarchy for the financial liabilities carried at fair value

As of December 31, 2010 and 2011, and June 30, 2012 (unaudited), the fair value hierarchy for the Company's financial assets and liabilities that are carried at fair value are as follows:



	December 31, 2010								December 31, 2011								June 30, 2012
																	(unaudited)
(in thousands)	Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3		Total		Level 1		Level 2		Level 3		Total
Assets
Money market funds	$	—	$	—	$	—	$	—	$	—	$	—	$	—	$	—	$	7,501	$	—	$	—	$	7,501
Certificates of deposits	—		—		—		—		—		—		—		—		995		—		—		995
Commercial paper	—		—		—		—		—		—		—		—		53,734		—		—		53,734
U.S. government and agency securities	—		—		—		—		—		—		—		—		9,991		—		—		9,991
Total assets measured at fair value	$	—	$	—	$	—	$	—	$	—	$	—	$	—	$	—	$	72,221	$	—	$	—	$	72,221

Liabilities
Convertible preferred stock warrants	$	—	$	—	$	1,127	$	1,127	$	—	$	—	$	1,853	$	1,853	$	—	$	—	$	—	$	—
Total liabilities measured at fair value	$	—	$	—	$	1,127	$	1,127	$	—	$	—	$	1,853	$	1,853	$	—	$	—	$	—	$	—

Fair value of the convertible preferred stock warrant liability

The changes in the fair value of the convertible preferred stock warrant liability is as follows:




	Years ended December 31,					Six months ended June 30,
(in thousands)	2010		2011			2012
						(unaudited)
Fair value at beginning of period	$	802	$	1,127		$	1,853
Change in fair value	325		981			1,631
Exercise of convertible preferred stock warrants	—		(255		)	(343		)
Conversion of preferred stock warrants to common stock warrants	—		—			(3,141		)
Fair value at end of period	$	1,127	$	1,853		$	—

- Definition

Tabular disclosure of assets, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, by class that are measured at fair value on a recurring and/or nonrecurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3).

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- Definition

Tabular disclosure of the fair value measurement of liabilities using significant unobservable inputs (Level 3), a reconciliation of the beginning and ending balances, separately presenting changes attributable to the following: (1) total gains or losses for the period (realized and unrealized), segregating those gains or losses included in earnings (or changes in net assets), and gains or losses recognized in other comprehensive income (loss) and a description of where those gains or losses included in earnings (or changes in net assets) are reported in the statement of income (or activities); (2) purchases, sales, issues, and settlements (each type disclosed separately); and (3) transfers in and transfers out of Level 3 (for example, transfers due to changes in the observability of significant inputs) by class of liability.

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