[1] Univ Hosp N Norway, Tromso, Norway
[2] Univ Tromso, Dept Neurol & Clin Neurophysiol, Tromso, Norway
[3] DNA Diagnost Lab, GSTS Pathol, London, England
[4] Univ Nottingham Hosp, Nottingham NG7 2UH, England
[5] ICH, Dubowitz Neuromuscular Ctr, London, England
[6] Newcastle Univ, Newcastle Upon Tyne, Tyne & Wear, England
[7] Kings Coll London, London WC2R 2LS, England
[8] St Thomas Hosp, Evelina Childrens Hosp, London, England