1. Howard Hughes Medical Institute, Department of Molecular Physiology and Biophysics, Department of Neurology, Department of Internal Medicine, University of Iowa Roy J. and Lucille A. Carver College of Medicine, 4283 Carver Biomedical Research Building, 285 Newton Road, Iowa City, IA 52242–1101, USA.
2. Dubowitz Neuromuscular Centre, Division of Neuroscience, Institute of Child Health and Great Ormond Street Hospital for Children, London, UK.
3. Department of Human Genetics, David Geffen School of Medicine, University of California, Los Angeles, CA 90095, USA.
4. Medical Nuclear Magnetic Resonance Facility, University of Iowa Roy J. and Lucille A. Carver College of Medicine, B291 Carver Biomedical Research Building, 285 Newton Road, Iowa City, IA 52242–1101, USA.