Department of Neuromuscular Diseases (E. Bugiardini, O.V.P, A.H., H.H., 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; Mitochondrial Medicine Group (E. Bottani, C.B., M.Z.), Medical Research Council Mitochondrial Biology Unit, Cambridge, United Kingdom; Department of Molecular and Translational Medicine (E. Bottani, A.V.), University of Brescia; Medical Genetics and Neurogenetics Unit (S.M., E.L., C.L.), Fondazione IRCCS Istituto Neurologico, "C. Besta," Milan, Italy; Neurogenetics Unit (C.W.), and Neurometabolic Unit (A.L., I.H., A.C.), The National Hospital for Neurology and Neurosurgery; Division of Neuropathology (K.V., J.L.H.), UCL Queen Square Institute of Neurology; and Dubowitz Neuromuscular Centre (R.Q.), Great Ormond Street Hospital, London, United Kingdom.