In the Ph.D.™ phage display system each copy of pIII displays the encoded peptide sequence. For this reason any insert that interferes with phage particle assembly or pIII function will not make viable phage. Typically phagemid systems, where a second copy of native coat protein is provided on by helper phage, are used to display protein-sized inserts on M13. We consider 25-50 amino acids to be the maximum range for this system. To some extent it is also sequence and secondary structure dependent.