1. Département de Differentiation Cellulaire, Centre de Génétique Moléculaire du CNRS, 91190 Gif sur Yvette, France
2. AFRC Institute of Animal Physiology, 307 Huntingdon Road, Cambridge CB3 OJQ, UK
3. Department of Anatomy, University of Cambridge, Downing Street, Cambridge CB2 3DY, UK