1 Department of Neuromuscular Diseases (O.V.P., A.H., E.B., R.Q., M.G.H., R.D.S.P.), UCL Queen Square Institute of Neurology and The National Hospital for Neurology and Neurosurgery, London, United Kingdom; Wellcome Centre for Mitochondrial Research (S.A.H., R.W.T.), Translational and Clinical Research Institute, Newcastle University, Newcastle Upon Tyne; Neurogenetics Unit (C.E.W.), and Neurometabolic Unit (I.P.H.), The National Hospital for Neurology and Neurosurgery; Division of Neuropathology (A.M.), UCL Queen Square Institute of Neurology; Department of Histopathology (A.M.), Camelia Botnar Laboratory, Great Ormond Street Hospital; and Dubowitz Neuromuscular Centre (R.Q.), Great Ormond Street Hospital, London, United Kingdom.