System.Reflection namespace not available when I target .NET 4.0

I had to upgrade to .NET 4.5 recently for a project, but I still need to target 4.0.

However when I do this, the System.Reflection namespace is no longer available.

According to the documentation here, it is part of version 4.0.

The System.Reflections.Context assembly just isn't there when I try to add a reference.

Can somebody tell me what might be going on here please?

The upgrade was automatically done when I opened a 4.5 solution, so I don't know if that gives any clues.