We thank Philip Leder (Harvard Medical School, Boston), Daniele Rotin (University of Toronto) and Marius Sudol (Mount Sinai Medical Center, New York) for generously providing the constructs encoding the WW domains. The work is supported by grants from the Deutsche Forschungsgemeinschaft (SFB415, project A9, to O.J.) and the University of Kiel (IZKF, project B4, to O.J.). Q.Z. was supported by a fellowship from the Chinese Scholarship Council. We thank Dana Becker for expert technical assistance.