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. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA, 02142, USA
5. Computer Laboratory, Cambridge, University of Cambridge, Cambridge, CB3 0FD, UK
6. European Bioinformatics Institute, European Molecular Biology Laboratory, Cambridge, CB10 1SD, UK