1. Harvard-MIT Division of Health Sciences and Technology, MIT, Cambridge, Massachusetts 02139, The David H. Koch Institute for Integrative Cancer Research, MIT, Cambridge, Massachusetts 02139, Department of Biology, MIT, Cambridge, Massachusetts 02139, Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts 02139, Division of Medicine, Brigham and Women’s Hospital, Boston, Massachusetts 02115, and Howard Hughes Medical Institute, Cambridge, Massachusetts 02139