Value Types do not inherit from any other type. The boxed type associated with a value type is an object type, and as such must specify either implicitly or explicitly what object type is its base type, i.e. the object type from which it inherits. The base object type that the boxed type derives from must have no fields defined. A boxed type is sealed so no other type can inherit from it.
CLS Rule 22: The CLS restricts boxed value types to inherit directly from System.ValueType unless they are enumerations, in which case they must inherit directly from System.Enum.
Note: In V1, the CLS restriction listed above is directly enforced by the VES.