1 Wellcome Centre for Mitochondrial Research, Institute of Genetic Medicine, Centre for Life (MJK, HES, RH, PFC), and Institute of Neuroscience (SRJ, MRB), Newcastle University; Departments of Neurology (MJK, SRJ, HES, RH, PFC, MRB) and Neurophysiology (SRJ, MRB), Royal Victoria Infirmary, Newcastle Upon Tyne; and Department of Clinical Neurosciences (MJK, PFC), University Neurology Unit, Cambridge Biomedical Campus, UK.