Hexa His is an amino acid sequence consisting of 6 His residues in a row, it is also known as hexa histidine-tag, 6xHis-tag, and by the trademarked name His-tag. Hexa His Responsible for the degradation of GM2 gangliosides, and a variety of other molecules containing terminal N-acetyl hexosamines, in the brain and other tissues. The form B is active against certain oligosaccharides. The form S has no measurable activity. Hexa His acts as a metal binding site for a recombinant protein. His-Tag sequence can be placed on the N- or -terminal of a target protein by using vectors from various commercial molecular biology companies. And this Tag is often followed by a cleavage site for a specific protease - for example Leu-Val-Arg-Gly-Ser peptide sequence is recognized and cleaved by the protease known as thrombin.

References: 1.  Hengen, P (1995). "Purification of His-Tag fusion proteins from Escherichia coli". Trends in Biochemical Sciences 20 (7): 285–6.