A polyhistiine-tag is an amino acid motif in proteins that consists of at least six histidine (His) residues, often at the N- or C-terminus of the protein. It is also known as hexa histidine-tag, 6xHis-tag, and by the trademarked name His-tag (registered by EMD Biosciences). The tag was invented by Roche and its vectors are distributed by Qiagen. A Variety of purification kits for histidine-tagged proteins are available from Qiagen, Sigma, Thermo Scientific, GE Healthcare and others.