1. Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, Massachusetts 02142
2. Department of Biochemistry and Chemistry, and of Chemical Pathology and Human Metabolism, Royal Free Hospital School of Medicine, Rowland Hill Street, London NW3 2PF, United Kingdom
3. Department of Biochemistry, Imperial College, London SW7 2AZ, United Kingdom
4. Department of Biochemistry, Dartmouth Medical School, Hanover, New Hampshire 03756
5. Department of Biology, Massachusetts Institute of Technology, Cambridge 02139.