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, UK. Last updated August 2004 by Professor E R Maher , Professor of Medical Genetics, Department of Paediatrics and Child Health, University of Birmingham Medical School, Birmingham, UK.