1 1Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02142, 2Department of Pathology, Beth Israel Deaconess Medical Center, 3Center for Biomedical Informatics, Harvard Medical School, 10 Shattuck Street, Boston, MA 02115, USA, 4Department of Biology, 5Howard Hughes Medical Institute, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139 and 6Department of Cell and Developmental Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655, USA (At 2014-05-13)