[1] Univ Toronto, Banting & Best Dept Med Res, Toronto, ON M5G 1L6, Canada
[2] Univ Toronto, Dept Mol & Med Genet, Toronto, ON M5G 1L6, Canada
[3] UCHSC B121, Dept Biochem & Mol Genet, Denver, CO 80262 USA
[4] Univ Virginia, Dept Microbiol, Charlottesville, VA 22901 USA
[5] Univ London Imperial Coll Sci Technol & Med, Dept Biochem, London SW7 2AZ, England
[6] Mem Sloan Kettering Canc Ctr, New York, NY 10021 USA