A library is provided having a plurality of recombinant derivatives of the SH3 domain of the Fyn kinase of SEQ ID NO: 1, where one or more of the derivatives have a specific binding affinity to a protein or peptide that is not a natural SH3 binding ligand. Substantially each of the derivatives of the SH3 domain of the Fyn kinase of SEQ ID NO: 1 has an amino acid sequence with at least 80% sequence identity to the amino acid sequence of SEQ ID NO: 1, and at least 90% identity to the amino acid of SEQ ID NO: 1 outside the src and RT loops. Additionally, the amino acid sequence has at least one amino acid in or positioned up to two amino acids adjacent to the RT loop or the src loop of SEQ ID NO: 1 which is substituted, deleted or added.