1. Program in Molecular Medicine, Hospital for Sick Children, Toronto, Ontario M5G 0A4, Canada.
2. Department of Biochemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
3. Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
4. Department of Chemistry, University of Toronto, Toronto, Ontario M5S 1A8, Canada.
5. Department of Biochemistry, McGill University, Montreal, Quebec H3G 1Y6, Canada.
6. Goodman Cancer Research Centre, McGill University, Montreal, Quebec H3A 1A3, Canada.