SHEILA A. STEWART1,7, DEREK M. DYKXHOORN2,7, DEBORAH PALLISER2,7, HANA MIZUNO1, EVAN Y. YU5, DONG SUNG AN6, DAVID M. SABATINI1,4, IRVIN S.Y. CHEN6, WILLIAM C. HAHN5, PHILLIP A. SHARP2,3,4, ROBERT A. WEINBERG1,4, and CARL D. NOVINA2 1Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142, USA 2Center for Cancer Research, 3McGovern Institute for Brain Research, and 4Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA 5Department of Medical Oncology, Dana-Farber Cancer Institute, and Departments of Medicine, Brigham and Women’s Hospital and Harvard Medical School, Boston, Massachusetts 02115, USA 6University of California, Los Angeles, AIDS Institute, David Geffen School of Medicine, Department of Microbiology, Immunology, Molecular Genetics & Medicine, Los Angeles, California 90025, USA