[1] Hammersmith Hosp, Dept Paediat, Imperial Coll Sch Med, London W12 0HN, England
[2] Hammersmith Hosp, Dubowitz Neuromuscular Ctr, Imperial Coll Sch Med, London W12 0HN, England
[3] GH Pitie Salpetriere, INSERM UR 153, Paris, France
[4] NE Wales Inst, MRIC Biochem Grp, Plas Coch Wrexham, Wales