1. Department of Oncology, University of Cambridge, Hutchison/MRC Research Centre, Cambridge Biomedical Campus, Cambridge CB2 0XZ, U.K.
2. Peterhouse, Cambridge CB2 1RD, U.K.
3. Wellcome Trust/MRC Centre for Stem Cell Research, University of Cambridge, Tennis Court Road, Cambridge CB2 1QR, U.K.
4. Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, U.K.