1. Department of Cellular and Molecular Neuroscience, Faculty of Medicine, Imperial College London, Hammersmith Hospital, Du Cane Road, London W12 0NN, U.K.
2. The Dubowitz Neuromuscular Centre, Institute of Child Health, University College London, 30 Guilford Street, London WC1N 1EH, U.K.
3. Royal Veterinary College, Hawkshead Campus, Hawkshead Lane, Hatfield, Herts. AL9 7TA, U.K.