1. MRC Laboratory of Molecular Biology, Francis Crick Avenue, Cambridge Biomedical Campus, Cambridge CB2 0QH, United Kingdom
2. Department of Molecular Biology and Center for Computational and Integrative Biology, Howard Hughes Medical Institute, Massachusetts General Hospital, Boston, Massachusetts 02114, United States
3. Department of Genetics, Harvard Medical School, Boston, Massachusetts 02115, United States
4. Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States