1. National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K.
2. Academic Department of Haematology, Royal Free Hospital School of Medicine, Pond Street, Hampstead NW3 2QG, U.K.
3. Membrane Molecular Biology Laboratory, Imperial Cancer Research Fund, Lincoln's Inn Fields, London WC2A 3PX, U.K.