1. Albert-Ludwigs-Universität Freiburg, Institut für Biologie I, Hauptstrasse 1, D-79104 Freiburg, Germany
2. Department of Physiology, UCSF, Third and Parnassus Avenue, San Francisco, CA 94143-0725, USA
3. Unit on Vertebrate Neural Development, NIH/NICHD Lab of Molecular Genetics, Building 6B, Room 3B-315, 9000 Rockville Pike, Bethesda MD 20892, USA
4. IGBMC, 1, rue Laurent Fries, BP163, 67404 Illkirch Cedex, France
5. Department of Cell Biology, Harvard Medical School, 240 Longwood Avenue, Boston, MA 02115, USA