1. Wellcome Trust/Cancer Research UK Gurdon Institute, University of Cambridge, Cambridge, United Kingdom
2. Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
3. Institute of Biology of Ecole Normale Supérieure (IBENS), Centre National de la Recherche Scientifique, UMR 8197, Paris, France
4. Institut National de la Santé et de la Recherche Médicale U 1024, Paris, France