A polyhistidine-tag is an amino acid motif in proteins that consists of at least five histidine (His) residues, often at the N- or C-terminus of the protein. Monoclonal antibodies specific to six histidine tags can greatly improve the effectiveness of several different kinds of immunoassays, helping researchers identify, detect, and purify polyhistidine fusion proteins in bacteria, insect cells, and mammalian cells.