Medical text written December 1996 by Contact a Family. Approved December 1996 by Professor M Patton, Professor of Medical Genetics, St George's Hospital Medical School, London, UK. Last updated September 2007 by Professor G Evans, Professor of Medical Genetics, Regional Genetics Service, St Mary's Hospital, Manchester, UK. Information on the Psychological and Behavioural Characteristics provided by Professor J Turk. Last updated December 2005 by Professor J Turk, Professor of Developmental Psychiatry and Consultant Child & Adolescent Psychiatrist, Department of Clinical Developmental Sciences, St. George's Hospital Medical School, London, UK.