1. Oslo University Hospital, Unit for Congenital and Inherited Neuromuscular Disorders, Oslo, Norway
2. University of Oslo, Faculty of Medicine, Oslo, Norway
3. University Hospital of North Norway, Department of Medical Genetics, Tromso, Norway
4. Oslo University Hospital, Division of Orthopaedic Surgery, Section of Research, Oslo, Norway
5. Oslo University Hospital, Department of Clinical Neurosciences for Children, Oslo, Norway