1. Banting and Best Department of Medical Research and Department of Molecular and Medical Genetics, University of Toronto, Toronto, Ontario, Canada M5G 1L6.
2. Department of Genome Sciences and Department of Medicine, University of Washington, Box 357730, Seattle, WA 98195, USA.
3. Department of Biology, University of Rome Tor Vergata, Via Della Ricerca Scientifica, 00133 Rome, Italy.
4. Samuel Lunenfeld Research Institute, Mount Sinai Hospital, 600 University Avenue, Toronto, Ontario, Canada M5S 1A8 and Department of Biochemistry, University of Toronto, Ontario, Canada M5G 1L6.
5. Department of Biology, Queens University, Kingston, Ontario, Canada K7L 3N6.
6. Howard Hughes Medical Institute, University of Washington, Box 353360, Seattle, WA 98195, USA.