1. C. E. Birse, B. A. Lee, N. J. Proudfoot, Sir William Dunn School of Pathology, South Parks Road, University of Oxford, Oxford OX1 3RE, UK.
2. L. Minvielle-Sebastia and W. Keller, Department of Cell Biology, Biozentrum, University of Basel, Klingelbergstrasse 70, CH-4056 Basel, Switzerland.