Medical text written November 1991 by Contact a Family. Approved November 1991 by Professor M Patton, Professor of Medical Genetics, St Georges Hospital Medical School, London, UK and Dr J E Wraith, Consultant Paediatrician, Royal Manchester Children's Hospital, Manchester, UK. Last updated August 1999 by Dr David Hilton-Jones. Last reviewed January 2004 by Dr David Hilton-Jones, Clinical Director, Muscle and Nerve Centre, Radcliffe Infirmary, Oxford, UK.