1. Protein Phosphorylation Laboratory, Imperial Cancer Research Fund, Lincoln's Inn Fields, London WC2A 3PX, U.K.
2. Consejo Superior de Investigaciones Cientifica, Universidad Complutense de Madrid, Instituto de Bioquimica, Facultad de Farmacia, 28040 Madrid, Spain
3. Royal Veterinary College, Royal College Street, London NW1 0TU, U.K.
4. Electron Microscopy Unit, Imperial Cancer Research Fund, Lincoln's Inn Fields, London WC2A 3PX, U.K.