The his-tag is a series of six to nine histidine residues generally fused to either the carboxy or amino terminus of a recombinant protein. The small size of the his-tag, compared with other common epitope tags, makes it less likely to obstruct the target protein’s structure or function and more suitable to use under denaturing conditions. The string of histidine residues binds to several types of immobilized metal ions, including nickel, cobalt and copper. The binding to metal ions under specific buffer conditions, allows for the simple purification and detection of his-tagged proteins.