n two-step affinity-tagged protein purification, a protein is first purified by affinity chromatography, then desalted. In some medium pressure chromatography systems, such as the NGC medium pressure chromatography systems, these two steps can be automated. In the first step, a recombinant protein mixture is passed over a chromatography support containing a ligand that selectively binds proteins that contain an affinity-tag sequence (typically His or GST). Contaminants are washed away, and the bound protein is then eluted in pure form.