1. A.-C. Butty and M. Peter, Swiss Institute for Experimental Cancer Research (ISREC), Chemin des Boveresses 155, 1066 Epalinges/VD, Switzerland. P. M. Pryciak, Department of Molecular Genetics and Microbiology, University of Massachusetts Medical Center, Worcester Foundation Campus, 222 Maple Avenue, Shrewsbury, MA 01545, USA. L. S. Huang and I. Herskowitz, Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94143–0448, USA.