1. Département de Biologie Cellulaire, Institut Jacques Monod, CNRS, Université Paris 7, 75251 Paris Cedex 05, France, Departments of Medicine and of Anatomy and Cell Biology, College of Physicians and Surgeons, Columbia University, New York, New York 10032, and Department of Anatomy and Cell Biology, University of Alberta, Edmonton, Alberta, Canada T6G 2H7