1. MRC Laboratory of Molecular Biology, Cambridge CB2 0QH, UK.
2. Whitehead Institute for Biomedical Research, Cambridge, MA 02142, USA.
3. Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02142, USA.
4. Howard Hughes Medical Institute, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
5. Koch Institute for Integrative Cancer Research, Cambridge, MA 02139, USA.
6. Broad Institute of MIT and Harvard, Cambridge, MA 02142, USA.