Institution: | (1) MRC Cellular Immunology Unit, Sir William Dunn School of Pathology, University of Oxford, South Parks Road, OX1 3RE Oxford, UK;(2) National Institute for Medical Research, The Ridgeway, Mill Hill, NW7 1AA London, UK;(3) Present address: MRC Laboratory of Molecular Biology, University Postgraduate Medical School, Hills Rd., CB2 2OH Cambridge, UK |