1. Department of Genetics, Max-Planck-Institut für Entwicklungsbiologie, Tübingen, Germany.
2. Current address: Department of Dermatology, University of Zürich Hospital, Zürich, Switzerland.
3. Current address: Interfakultäres Institut für Biochemie, Universität Tübingen, Tübingen, Germany.
4. Department of Chemistry and Biochemistry, University of California, Los Angeles, California.
5. Current address: Orthopaedic Research Laboratories, Harvard Medical School, Boston, Massachusetts.