¹ Cancer Program, Broad Institute of Massachusetts Institute of Technology and Harvard University, Cambridge Center, Cambridge, MA 02142, USA

² Department of Pediatric Oncology, Dana-Farber Cancer Institute and Harvard Medical School, Binney Street, Boston, MA 02115, USA

³ Department of Chemistry, Massachusetts Institute of Technology, Massachusetts Avenue, Cambridge, MA 02139, USA

⁴ Departments of Biological Sciences and Chemistry, Columbia University, Fairchild Center MC2406, Amsterdam Avenue, New York, NY 10027, USA

⁵ Howard Hughes Medical Institute, Jones Boulevard, Chevy Chase, MD 20815, USA

⁶ Current address: Advanced Genetic Analysis, Applied Biosystems, Cummings Center, Beverly, MA 01915, USA