1. Department of Biological Sciences, Imperial College of Science, Technology and Medicine, London SW7 2AY, U.K.
2. Institute of Genetics, Queen's Medical Centre, University of Nottingham, Nottingham NG7 2UH, U.K.
3. Neuromuscular Research Department, Institute of Anatomy, University of Vienna, Waehringerstrasse 13, 1090 Vienna, Austria
4. National Institutes of Health, Building 10, 9000 Rockville Pike, Bethesda, MD 20892-1260, U.S.A.