1. Howard Hughes Medical Institute (HHMI) Summer Institute, Marine Biological Laboratory, Woods Hole, MA 02543, USA.
2. Department of Cellular and Molecular Pharmacology and Howard Hughes Medical Institute, University of California, San Francisco, CA 94158, USA.
3. Department of Biophysics and Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX 75390, USA.
4. HHMI Mass Spectrometry Laboratory and Department of Molecular and Cellular Biology, University of California, Berkeley, CA 94720, USA.