1. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
2. Department of Chemistry, University of California, Berkeley, CA 94703, USA.
3. Department of Biochemistry and Biophysics, University of California, San Francisco, CA 94143, USA.
4. Department of Molecular and Cell Biology, University of California, Berkeley, CA 94703, USA.
5. Whitehead Institute for Biomedical Research, Howard Hughes Medical Institute, and Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02138, USA.
6. Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, USA.