The amino acid sequence consisting of 6 His residues in a row, called His-Tag, would act 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.