1. Division of Protein and Nucleic Acid Chemistry, Medical Research Council Laboratory of Molecular Biology Hills Road Cambridge CB2 0QH UK
2. Section of Structural Biology, Institute of Cancer Research 237 Fulham Road London SW3 6JB UK
3. Department of Biochemistry, Emory University School of Medicine 4017 Rollins Research Building, 1510 Clifton Road Atlanta 303222 Georgia USA