A<unk>  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.
The use of the tag for academic users is unrestricted; however, commercial users must pay royalties to Roche.  Suitable tag sequences are available for free commercial use; for example, MK(HQ)6 may be used for enhanced expression in E. coli and tag removal. The total number of histidine residues may vary in the tag. The his-tag may also be followed by a suitable amino acid sequence that facilitates a removal of the<unk>  using endopeptidases. This extra sequence is not necessary if exopeptidases are used to remove N-terminal His-tags (e.g., Qiagen TAGZyme).  Furthermore, exopeptidase cleavage may solve the unspecific cleavage observed when using endoprotease-based tag removal.<unk> s are often used for affinity purification of genetically modified proteins.
Polyhistidine-tag
Triage tag
CARD domain
Ankyrin repeat
Foldamers
Tandem mass tags
SUMO protein
Isotopomers