The invention relates to a method for modifying one or more peptide ligands, comprising polypeptides covalently linked to a molecular scaffold at two or more amino acid residues, comprising the steps of providing one or more peptide ligands, wherein the polypeptide comprises two or more reactive groups which form a covalent linkage to the molecular scaffold, and at least one loop which comprises a sequence of two or more amino acids subtended between two of said reactive groups； exposing the peptide ligands to one or more proteases； and sorting the ligands according to the extent of proteolytic cleavage.
