Medical text written November 1991 by Contact a Family. Approved November 1991 by Professor M Patton, Professor of Medical Genetics, St George’s Hospital Medical School, London, UK and Dr JE Wraith, Consultant Paediatrician, Royal Manchester Children’s Hospital, Manchester, UK. Psychological and behavioural characteristics text written by Dr O Udwin, Consultant Clinical Psychologist, West London Mental Health Trust, London, UK. Last updated January 2010 by Dr J Whittington, Senior Research Associate, Section of Developmental Psychiatry, Department of Psychiatry, University of Cambridge, Cambridge, UK.