1. Wellcome Trust, Cancer Research UK, Gurdon Institute, University of Cambridge, Cambridge CB2 1QN, UK.
2. Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG, UK.
3. Wellcome Trust-Medical Research Council, Stem Cell Institute, University of Cambridge, University of Cambridge, Cambridge CB2 3DY, UK.
4. Department of Biochemistry and Molecular Biology, University of Iceland, Vatnsmyrarvegi 16, 101 Reykjavík, Iceland.