1. Department of Biology, University College London, Gower Street, London WC1E 6BT, UK.
2. Zoologisches Institut, Universität Zürich, Winterthurerstrasse 190, 8057 Zürich, Switzerland.
3. Growth Regulation Laboratory, Imperial Cancer Research Fund, Post Office Box 123, 44 Lincoln's Inn Fields, London WC2A 3PX, UK.