Filamentous phage display systems are generally based on N-terminal fusions to the coat proteins pIII or pVIII. pIII is present at 5 copies per virion, of which all 5 can be fused to short peptides without interfering with phage infectivity. The major coat protein pVIII is present at ~2700 copies per virion, of which ~10% can be reliably fused to peptides or proteins. As a result, peptides expressed as pIII fusions are present at low valency (1-5 copies per virion), while pVIII fusions are present at high valency (~200 copies per virion). The increased avidity effect of high valency pVIII display permits selection of very low affinity ligands, while low valency pIII display limits selection to higher affinity ligands. The Ph.D.™ Phage Display Cloning system is designed to make pIII fusions (5 copies of the peptide per virion). NEB’s pre-made libraries are the pentavalent pIII display format.