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. Last updated August 2006 by Professor A T Moore, Duke-Elder Professor of Ophthalmology, Institute of Ophthalmology, London, UK.