1 Department of Neurology, Washington University School of Medicine, Box 8111, 660 South Euclid Avenue, Saint Louis, MO 63110, USA. Electronic address: weihlc@neuro.wustl.edu.
2 Tampere Neuromuscular Center and Folkhalsan Genetic Institute, Helsinki, Finland.
3 UCL Institute of Neurology, Queen Square, London, UK.