A his-tag consists of approximately six histidine residues near the N- or C-Terminus of a Protein. The total number of histidine residues may vary. It may also be followed by a suitable amino acid sequence that facilitates a removal of the his-tag.
The his-tag can be used in affinity chromatography together with a column that has nickel bound to it. Histidine is a good ligand for nickel so the fusion protein will bind to the column. The protein can be eluted with a solution of imidazole.