1. Whitehead Institute for Biomedical Research, Cambridge, Massachusetts 02142, United States
2. Department
of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, United States
3. Department
of Gastroenterology, Massachusetts General Hospital, 185 Cambridge
Street, Boston, Massachusetts 02114, United States
4. Department
of Systems Biology, Harvard Medical School, 200 Longwood Avenue, Boston, Massachusetts 02115, United States