1. Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California 92093, United States
2. Department of Bioengineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
3. Electrical Engineering and Computer Science, Massachusetts Institute Technology, Cambridge, Massachusetts 02139, United States, and Division of Medicine, Brigham and Women's Hospital, Boston, Massachusetts 02115, United States
4. Howard Hughes Medical Institute, Cambridge, Massachusetts 02139, United States