The hexahistidine tag (6× His-tag) is the most frequently used affinity tag for protein enrichment. His-tagged proteins can be purified easily by the chelated metal ions as affinity ligands. The basis for affinity purification is known as immobilized metal affinity chromatography (IMAC). His-tag can bind best to IMAC resin in near-neutral buffer conditions (physiologic pH and ionic strength), and thus the fusion proteins can be eluted with binding buffer containing certain concentrations of imidazole. If possible, the elution is also accomplished with low pH (e.g., 0.1 M glycine-HCl, pH 2.5) or an excess of strong chelator (e.g., EDTA).