1. Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, UK
2. Wellcome Trust/Cancer Research UK Gurdon Institute, University of Cambridge, Cambridge CB2 1QN, UK
3. Wellcome Trust/Medical Research Council Stem Cell Institute, University of Cambridge, Cambridge CB2 1QR, UK
4. Howard Hughes Medical Institute and the Department of Molecular and Cell Biology, University of California, Berkeley, CA 94720, USA