The present invention relates to a recombinant binding protein comprising at least one derivative of the Src homology 3 domain (SH3) of the FYN kinase, wherein at least one amino acid in or positioned up to two amino acids adjacent to the src loop and/or at least one amino acid in or positioned up to two amino acids adjacent to the RT loop is substituted, deleted or added. Furthermore, the Invention is directed to fusion proteins comprising a binding protein according to the invention fused to a pharmaceutically and/or diagnostically active component. In addition, the invention concerns nucleotides coding for these binding and/or fusion proteins as well as corresponding vectors and host calls. Moreover, the invention also concerns a method for co-crystallizing said binding and corresponding target proteins and a method for purifying and/or enriching proteins by contacting a binding protein of the invention with a corresponding target protein. Last but not least, the present invention relates to the use of binding and/or fusion proteins of the present invention for preparing a medicament or a diagnostic means as well as to pharmaceutical or diagnostic compositions comprising said binding and/or fusion proteins.
