The top after-market NASDAQ gainers Tuesday were: On Assignment, FSI International, Lexicon Pharmaceuticals, Anthera Pharmaceuticals, Warren Resources, Vical Inc, K-Swiss, Ambassadors Group, American Superconductor and Idenix Pharmaceuticals.

On Assignment Inc. (ASGN) stock climbed 24.27 percent to $17.0 in aftermarket session. The company announced that it has signed a definitive agreement to acquire Apex Systems Inc for $600 million. On Assignment also said the acquisition is expected to be significantly accretive to On Assignment's earnings per share, on both a GAAP and cash EPS basis, in 2012 and beyond.

FSI International Inc. (FSII) stock climbed 14.13 percent to $5.25 in post market trading session. The company reported second quarter net income of $3.7 million or $0.09 per share compared to $4.9 million or $0.13 per share in the same period last year and against analysts’ estimation of $0.05 per share.

Lexicon Pharmaceuticals Inc. (LXRX) stock climbed 9.09 percent to $2.04 in aftermarket trading. The company announced Tuesday that the US Food and Drug Administration (FDA) has granted its request for Orphan Drug designation for telotristat etiprate (LX1032) for the treatment of carcinoid syndrome.

Anthera Pharmaceuticals Inc. (ANTH) stock climbed 8.93 percent to $3.17 in post market trading session. The company stock plunged more than 4 percent in regular trading.

Warren Resources Inc. (WRES) stock surged 7.95 percent to $3.53 in post market trading session. The company stock plunged more than 6 percent in regular trading.

Vical Inc. (VICL) stock surged 7.87 percent to $3.29 in post market trading session.

K-Swiss Inc. (KSWS) stock surged 7.56 percent to $4.41 in aftermarket trading.

Ambassadors Group Inc. (EPAX) stock advanced 7.5 percent to 5.88 in post market trading. The company stock plunged more than 6 percent in regular trading.

American Superconductor Corp. (AMSC) stock rose 6.86 percent to $4.36 in post market session. The stock has a 52-week low of $3.21, a 52-week high of $25.19 and $209.98 million in market capitalization.

Idenix Pharmaceuticals Inc. (IDIX) stock gained 6.33 percent to $11.09 in aftermarket trading. The stock has a 52-week low of $2.74, a 52-week high of $15.25 and $1.12 billion in market capitalization.