1. Department of Biology, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139, USA.
2. Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02142, USA.
3. Broad Institute of MIT and Harvard, 7 Cambridge Center, Cambridge, MA 02142, USA.
4. David H. Koch Institute for Integrative Cancer Research at MIT, Cambridge, MA 02139, USA.
5. Howard Hughes Medical Institute, Department of Biology, MIT, Cambridge, MA 02139, USA.
6. Department of Systems Biology, Harvard Medical School, Boston, MA 02115, USA.