1. 1 From the Purine Research Unit, Guy's Hospital; the Department of Haematology, King's College Hospital, London; the Department of Haematology, Royal Hallamshire Hospital, Sheffield, United Kingdom; the Istituto di Biochimica, Facolta di Medicina, Universita di Ancona, Italy; the Department of Pediatrics, The National Hospital, Oslo, Norway; the Institute of Hematology, Chaim Sheba Medical Center, Tel Hashomer, Israel; and the Department of Chemical Pathology, University of Cape Town, South Africa.