1. Muscle Cell Biology GroupMRC Clinical Sciences CentreImperial College London W12 0NN UK
2. Centre for RheumatologyRoyal Free Hospital School of MedicineUniversity College London London NW3 2QG UK
3. Dubowitz Neuromuscular CentreDepartment of PaediatricsHammersmith HospitalImperial College London W12 0NN UK
4. Research Centre for Genetic MedicineChildren's National Medical Centre 111 Michigan Avenue, N.W. Washington, DC 20010 USA