1. School of Biochemistry and Immunology, Trinity College, Dublin 2, Ireland
2. Department of Medicine, University of Cambridge, Addenbrooke's Hospital, Cambridge CBQ 2QQ, England, UK
3. Henry Wellcome Laboratories for Medical Research, University of Sheffield, Sheffield S10 2TN, England UK
4. The Medical Research Council Laboratory of Molecular Biology, Cambridge CBQ 2QH, England, UK
5. Centro Nacional de Biotecnologia, Campus de Cantoblanco, 28049 Madrid, Spain