The histidine affinity tag consists of 6 histidine amino acid residues located at the N- or C- terminus of recombinant protein. Purification of the recombination protein involves the use of immobilized metal affinity chromatography, which takes advantage of the high affinity of the hexahistidine residues for transition metal ions (e.g Ni2+ (most common), Zn2+, Cu2+ or Co2+).  Other proteins present in the lysate or solution are unlikely to bind to the immobilized metal ions, thus allowing for the isolation of  the recombinant protein. To release the recombinant protein from the resin containing the immobilized metal ions, the pH is lowered or a strong chelating agent (e.g. imidazole or EDTA) is used. Compared to other purification tags, histidine tags are small, have low toxicity and low levels of immunogenicity.