PIII protein construct for phage libraries. The glycine and two proline residues at the carboxyl end of the random regions function as a flexible linker and may serve to discourage association of the random peptide with the native pIII protein. Structural constraints in the CL10 and CL6 libraries were imposed by disulfide bonds between the cysteine residues flanking the variable region. CL10 and CL6 also include at the amino terminus of the mutated pIII protein wild-type residues (glutamic acid and alanine) and may help to preserve the signal peptidase cleavage site allowing cleavage of the peptide backbone at the correct site on the pIII pre-protein.