1. A. Colavita, H. Zheng, J. G. Culotti, Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Ontario M5G 1X5, Canada, and Department of Molecular and Medical Genetics, University of Toronto, Ontario M5S 1A8, Canada. S. Krishna and R. W. Padgett, Waksman Institute, Department of Molecular Biology and Biochemistry, and the Cancer Institute of New Jersey, Rutgers University, Piscataway, NJ 08854–8020, USA.