1. Division of Immune Cell Biology, MRC National Institute for Medical Research, Mill Hill, London NW7 1AA, U.K.
2. Beatson Institute for Cancer Research, Bearsden, Glasgow G61 1BD, U.K.
3. Division of Molecular Structure, MRC National Institute for Medical Research, Mill Hill, London NW7 1AA, U.K.
4. Department of Microbiology, School of Medicine and National Emerging Infectious Disease Laboratories, Boston University, Boston, MA 02118, U.S.A.