Epitope tags are short peptide sequences which are chosen because high-affinity antibodies can be reliably produced in many different species. These are usually derived from viral genes, which explain their high immunoreactivity. They are particularly useful for western blotting, immunofluorescence and immunoprecipitation experiments, although they also find use in antibody purification. S1 tag is an epitope tag composed of a nine-residue peptide, NANNPDWDF, derived from the hepatitis B virus preS1 region.