Could not load file or assembly 'System.Web.Security.SingleSignOn,

Could not load file or assembly 'System.Web.Security.SingleSignOn, Version=1.0.0.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35' or one of its dependencies. The system cannot find the file specified.

I am getting above error. I am trying to do ADFS2.0 application. Targeted frame work in 4.0. This sample application was developed in VS 2010 and deployed in windows 2008 R2 server on which .net framework is 4.0 and ADFS2.0 was installed.  In assemblies I tried for 'System.Web.Security.SingleSignOn' dll but could not find out.