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. Updated November 2001 by Professor Geoff Lindsay and Professor Julie Dockrell, Institute of Education, London, UK. Last updated December 2005 by Professor Geoff Lindsay, Professor of Special Educational Needs and Educational Psychology, University of Warwick, Coventry, UK.