Affiliation: | a Depts of Medicine and of Anatomy and Cell Biology, College of Physicians and Surgeons, Columbia University, 630 West 168th Street, Box 83, New York, NY 10032, USA b Dept de Biologie Cellulaire, Institut Jacques Monod, Université Paris 7, Tour 43, 2 Place Jussieu, 75251, Paris, Cedex 05, France |