1. Contribution from the Protein Engineering Network Centers of Excellence and Departments of Molecular and Medical Genetics, Biochemistry and Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A8, Hospital for Sick Children, Biochemistry Research, Toronto, Ontario, Canada M5G 1X8, and Macromolecular NMR Section, ABL-Basic Research Program, NCI-Frederick Cancer Research and Development Center, Frederick, Maryland 21702-1201