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. Psychological and behavioural characteristics written November 1998 by Dr O Udwin, Consultant Clinical Child Psychologist, Society for the Study of Behavioural Phenotypes. Medical text last updated November 2004 and Psychological and behavioural characteristics last reviewed November 2004 by Dr J Whittington, Senior Research Associate, Section of Developmental Psychiatry, Department of Psychiatry, University of Cambridge, Cambridge, UK.