1. Wellcome Trust/CRUK Gurdon Institute, University of Cambridge, Cambridge, CB2 1QN, UK
2. Current address: Cold Spring Harbor Laboratory, 1 Bungtown Road, Cold Spring Harbor, NY11724, USA
3. School of Cellular and Molecular Medicine, Faculty of Biomedical Sciences, University of Bristol, Biomedical Sciences Building, University Walk, Bristol, BS8 1TD, UK
4. Wellcome Trust/MRC Stem Cell Institute and Department of Pathology, University of Cambridge, UK