The MAT tag stands for “metal affinity tag” and is a commercially developed epitope for the purification of fusion proteins by immobilized metal affinity chromatography (IMAC). It is a seven-residue peptide (HNHRHKH) with high affinities for nickel and cobalt. It can be engineered to the N- or C- termini of recombinant proteins. In addition, antibodies that target the MAT epitope can allow for immunological detection and analyses of MAT tagged proteins.