Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02142, Department of Pathology, Beth Israel Deaconess Medical Center, Center for Biomedical Informatics, Harvard Medical School, 10 Shattuck Street, Boston, MA 02115, USA, Department of Biology, Howard Hughes Medical Institute, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139 and Department of Cell and Developmental Biology, University of Massachusetts Medical School, 55 Lake Avenue North, Worcester, MA 01655, USA.