Affinity chromatography does not exploit the physical differences between proteins. Prior knowledge about the protein of interest will help determine the choice of a good ligand for affinity chromatography. Successful affinity chromatography depends upon the reversible binding of the target molecule to a ligand attached to a support resin. Large ligands can be attached directly to an affinity resin and still retain their attractiveness to the target molecule. Increasing the salt concentration to >1.0 M NaCl or decreasing the pH to 2.0 are non-specific elution procedures that can be applied. The stability of the coupling and leakage of the ligand from the resin is particularly important if the target protein is to be used therapeutically. Particularly, the atom used to attach the ligand to the resin will have restricted movement compared to the free ligand. The target protein's N-terminal addition can be used to purify the protein by binding and elution from an affinity resin.