1. R. Festenstein, M. Tolaini, P. Corbella, C. Mamalaki, D. Kioussis, Division of Molecular Immunology, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, UK.
2. J. Parrington, M. Fox, A. Miliou, Medical Research Council Human Biochemical Genetics Unit, Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK.
3. M. Jones, Nuffield Department of Pathologgy, John Radcliffe Hospital, Oxford OX3 9DU, UK.