Affinity-tag systems which enable efficient target protein expression and purification share the following features: (a) onestep adsorption purification; (b) a minimal effect on tertiary structure and biological activity; © easy and specific removal to produce the native protein; (d) simple and accurate assay of the recombinant protein during purification; (e) applicability to a number of different proteins. A polyhistidine-tag is an amino acid motif in proteins that usually consists of six histidine residues (hexa histidine-tag, 6×His-tag). 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 polyhistidine-tag using endoprotease. Poly histidine tags are often used for affinity purification of genetically modified proteins. A widely employed method utilizes immobilized metal affinity chromatography. Immobilized metal-affinity chromatography (IMAC) is based on the interaction between a transition metal ion (Co2+, Ni2+, Cu2+, Zn2+) immobilized on a matrix and specific amino acid side chains. Histidine is the amino acid that exhibits the strongest interaction with immobilized metal ion matrices. Following washing of the matrix material, peptides containing poly histidine sequences can be easily eluted by either adjusting the pH of the column buffer or by adding free imidazole.