Overview:

This polyhistidine epitope tag is generally comprised of six consecutive histidine amino acid residues located at the N-terminal, C-terminal, or internally [1].The 6 His-Tag is widely used because of its affinity to bind nickel or cobalt metal ions attached to sepharose, which can then be used to purify the protein in a native or denatured state [2].

References:

1.Jonathan, W. et al: Epitope tagging, Annual Review of Genetics, 1998; 32, 601-618 2.Schmeisser, H, et al:, Binding Characteristics of IFN-alpha Subvariants to IFNAR2-EC and Influence of the 6-Histidine Tag. J Interferon Cytokine Res. 2006; Dec. 26(12):866-76.