1. Mammalian Genetics Unit, Medical Research Council, Harwell Institute, Oxfordshire OX11 0RD, UK
2. Department of Biochemistry, St. Jude Children’s Research Hospital, Memphis, TN 38105, USA
3. Institute of Physiological Chemistry, University Medical Center Mainz, 55128 Mainz, Germany
4. Mary Lyon Centre, Medical Research Council, Harwell Institute, Oxfordshire OX11 0RD, UK