1. Wellcome Trust Centre for Stem Cell Research, University of Cambridge, Cambridge CB2 1QR, England, UK
2. Cancer Research Technology Ltd., Cancer Research UK, London WC2A 3NL, England, UK
3. Goethe University of Frankfurt, Frankfurt am Main 60325, Germany
4. Wellcome Trust/Cancer Research UK Gurdon Institute, Cambridge CB2 1QN, England, UK