NEW YORK, Dec. 22, 2011 /PRNewswire/ --WPX Energy Inc. (NYSE: WPXwi) will replace Compuware Corp. (NASD: CPWR) in the S&P 500 index, Compuware will replace Exterran Holdings Inc. (NYSE: EXH) in the S&P MidCap 400 index, and Exterran Holdings will replace Skyline Corp. (NYSE: SKY) in the S&P SmallCap 600 index after the close of trading on Friday, December 30. S&P 100 & 500 constituent The Williams Companies Inc. (NYSE: WMB) is spinning off WPX Energy in a transaction expected to be completed on December 31 pending final approvals. The Williams Companies stub will remain in the S&P 500 index. Compuware's market capitalization is below $1.9 billion making it more representative of the mid cap market space, and Exterran's market capitalization is below $600 million making it more representative of the small cap market space. Skyline is ranked #600 in the SmallCap 600 index.

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WPX Energy is an independent natural gas and oil exploration and production company. Headquartered in Tulsa, OK, the company will be added to the S&P 500 GICS (Global Industry Classification Standard) Oil & Gas Exploration & Production Sub-Industry index.

Compuware provides software and web performance solutions, professional services, and application services. Headquartered in Detroit, MI, the company will be added to the S&P MidCap 400 GICS Application Software Sub-Industry index.

Exterran Holdings provides operations, maintenance, service, and equipment for oil and natural gas production, processing, and transportation applications. Headquartered in Houston, TX, the company will be added to the S&P SmallCap 600 GICS Oil & Gas Equipment & Services Sub-Industry index.

Following is a summary of the changes:

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SOURCE S&P Indices