1. Wellcome Trust/Cancer Research UK Gurdon Institute, Cambridge, UK.
2. Department of Genetics, University of Cambridge, Cambridge, UK.
3. Department of Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720-3200, USA.
4. Howard Hughes Medical Institute, 4000 Jones Bridge Road, Chevy Chase, MD 20815, USA.
5. Biological Sciences and Engineering Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA.
6. California Institute for Quantitative Biosciences, Berkeley, CA 94720, USA.