The expression of a protein fused to a tag (epitope and fluorescent protein tag) is a successful strategy for the investigation of the role individual proteins in cell systems. Highly specific monoclonal and polyclonal antibodies to some of the most popular tag and fluorescent protein constructs have been assembled. All of these antibodies have been carefully validated for use in western blotting, in addition, some have also been validated for use in immunofluorescence microscopy.