The T7 tag is an 11 amino acid peptide encoded in the leader sequence of T7 bacteriophage gene10. This gene encodes a T7 major capsid protein whose function is not clear. The T7 tag serves as a tag in many expression vectors including the pET system that is based on the very efficient T7 RNA polymerase expression system. Monoclonal antibodies specific for T7 tag are an important tool for studying expression of recombinant T7-tagged proteins.