1. D. T. Fearon is at the Wellcome Trust Immunology Unit, Department of Medicine, University of Cambridge School of Clinical Medicine, Cambridge CB2 2SP, UK.
2. R. M. Locksley is in the Department of Medicine, University of California, San Francisco, Post Office Box 0654, C-443, 521 Parnassus Avenue, San Francisco, CA 94143- 0654, USA.