1. Faculté de Médecine et de Pharmacie, 34 rue du Jardin des Plantes, B.P. 199, 86005 Poitiers Cedex, France, Cell Cycle Group, Station Biologique, C.N.R.S., B.P. 74, 29682 Roscoff Cedex, Bretagne, France, INSERM U 383, Génétique, Chromosomes et Cancer, Hôpital Necker, 75015 Paris, France, and Laboratory of Molecular Biophysics, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom