1. Structural Biology Unit, Department of Experimental Oncology, European Institute of Oncology, Via Ripamonti 435, 20141 Milan, Italy, Department of Molecular and Cell Biology, Howard Hughes Medical Institute, University of California Berkeley, Berkeley, California 94720-3202, Department of Pathology, Harvard Medical School and Howard Hughes Medical Institute, 77 Avenue Louis Pasteur, Boston, Massachusetts 02115, and Cell Cycle Group, Station Biologique, B.P. 74, 29682 Roscoff Cedex, Bretagne, France