1. Institut de Génétique et de Biologie Moléculaire et Cellulaire (IGBMC), CNRS/INSERM/ULP, BP10142, Illkirch Cedex F-67404, France
2. Ecole Normale Supérieure de Paris, INSERM U 497, 75005 Paris, France
3. Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Cruz, CA 95064
4. Department of Medical Genetics, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3