1. B. A. Griffin, Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, CA 92093–0647, USA. S. R. Adams, Department of Pharmacology, University of California San Diego, La Jolla, CA 92093–0647, USA. R. Y. Tsien, Department of Pharmacology, Department of Chemistry and Biochemistry, and Howard Hughes Medical Institute, University of California San Diego, La Jolla, CA 92093–0647, USA.