1. Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, MA 02142, USA.
2. Department of Biology, Massachusetts Institute of Technology (MIT), Cambridge, MA 02139, USA.
3. MIT Laboratory of Computer Science, 200 Technology Square, Cambridge, MA 02139, USA.
4. Departments of Biochemistry and Molecular Biology, Medicine, and Human Genetics, University of Chicago, Chicago, IL 60637, USA.