1. Programme in Developmental Genetics, The Babraham Institute, Cambridge CB2 4AT, 1 and
2. Chromatin and Gene Expression Group, University of Birmingham Medical School, Birmingham B15 2TT, 2 United Kingdom;
3. Gene Research Center, Gunma University, Maebashi 371-8511, Japan 3 ; and
4. Institute of Molecular Genetics, CNRS UMR-5535, IRF-24, 34293 Montpellier Cedex 5, France4