1. Howard Hughes Medical Institute and the Department of Cellular and Molecular Pharmacology, University of California, San Francisco, 600 16th Street, San Francisco, CA 94158, USA.
2. Department of Molecular and Cellular Biology, University of California, Davis, One Shields Avenue, 3203 Life Sciences, Davis, CA 95616, USA.
3. Physiology Course, Marine Biological Laboratory, 7 MBL Street, Woods Hole, MA 02543, USA.