The present invention is directed to SH3 domain derivatives having a specific binding affinity to a target molecule. In this respect, the invention provides SH3 domain derivatives of nephrocystin (NPHP1) and the Tec kinase. The invention also provides a method for the production of a library comprising recombinant derivatives of NPHP1 or the Tec kinase SH3 domains and a method for selecting from the library one or more derivatives of the SH3 domain of nephrocystin (NPHP1) or the Tec kinase having a specific binding affinity to a target molecule.