Why is System.Web.Mvc not listed in Add References?

Question

Using C#, Visual Studio 2010.

There is a namespace called System.Web.Mvc documented on MSDN. The documentation for all the types in that namespace says that they are

Answer 1

In VS Express 2012 I couldn't find System.Web.Mvc in the "assemblies" tab, but after a bit of searching I found out that I need to look into "assemblies\extensions" tab rather than the default "assemblies\framework" tab.