1. Department of Molecular Biology, Massachusetts General Hospital, Boston, MA 02114, USA.
2. Department of Genetics, Harvard Medical School, Boston, MA 02115, USA.
3. Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, CA 94158, USA.
4. Chemistry and Chemical Biology Graduate Program, University of California, San Francisco, San Francisco, CA 94158, USA.