The His tag, also known as polyhistidine, 6xHis, or 6-his tag, is peptide sequence covalently connected to a protein of interest by means of recombinant expression. It is composed of six (or between 5 to 10) histidine residues (HHHHHH) and is typically attached to the N- or C-terminal ends of the protein. It is often used for affinity purification, utilizing the tag’s high affinities for nickel and cobalt. Additionally, specific antibodies can also be used to analyze his-tagged proteins.