Windows 8 RT: using System.Xml.XPath

In my Windows Store App project, I put in a "using System.Xml.XPath" statement. Even though I've added System.Xml to my project referenes, I still get the following compile error:

The type or namespace name 'XPath' does not exist in the namespace 'System.Xml' (are you missing an assembly reference?)

Any suggestions on how to get XPath to compile?

Thanks...

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Randy