[1] Guys Hosp, Diagnost Genet Lab, GSTS Pathol, London SE1 9RT, England
[2] Dubowitz Neuromuscular Ctr, ICH, London, England
[3] Univ Groningen, Univ Med Ctr Groningen, Dept Genet, Groningen, Netherlands
[4] Guys & St Thomas NHS Fdn Trust, Evelina Childrens Hosp, Dept Paediat Neurol, Neuromuscular Serv, London, England