1. Medical Sciences, Indiana University School of Medicine, Bloomington, Indiana 47405;
2. Department of Biochemistry and Molecular Biology, Department of Chemistry, and The Michael Smith Laboratories, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z3;
3. Department of Oncological Sciences, Huntsman Cancer Institute, University of Utah Health Sciences, Salt Lake City, Utah 84112;
4. Howard Hughes Medical Institute, Chevy Chase, Maryland 20815