T7 tag is an epitope tag composed of an 11-residue peptide encoded from the leader sequence of the T7 bacteriophage gene10, which encodes a T7 major capsid protein. The T7 tag is commonly engineered onto the N- or C- terminus of a protein of interest so that the tagged protein can be analyzed and visualized using immunochemical methods. The T7 tag has been used extensively as a general epitope tag in many expression vectors including the pET system that is based on T7 RNA polymerase expression systems.