1. European Molecular Biology Laboratory Meyerhofstrasse 1 69117 Heidelberg Germany
2. Department of Immunology and Oncology, Centro Nacional de Biotecnología, Consejo Superior de Investigaciones Científicas, UAM Campus de Cantoblanco E‐28049 Madrid Spain
3. Present address: 519 LSA, Molecular and Cellular Biology Department, U.C. Berkeley Berkeley CA 94720 USA
4. Deutsches Krebsforschungszentrum Im Neuenheimer Feld 280 69120 Heidelberg Germany
5. Regeneron Pharmaceuticals Inc. Tarrytown NY 10591 USA
6. Division of Liver Diseases and Department of Medicine, Mount Sinai School of Medicine New York NY 10029 USA
7. Present address: Howard Hughes Medical Institute, Department of Genetics, Harvard Medical School Boston MA 02115 USA
8. Present address: LSGC–CNRS F‐54505 Vandoeuvre‐les‐Nancy France
9. Molecular Neurobiology Laboratory, Salk Institute for Biological Studies La Jolla CA 92037 USA