1. Howard Hughes Medical Institute, University of California San Diego, La Jolla, California 92093-0365, USA
2. Department of Mathematics and Mathematical Center for Interdiscipline Research, Soochow University, 1 Shizi Street, Suzhou, 215006 Jiangsu, China
3. Department of Mathematics and Quantitative Biology Graduate Program, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093-0112, USA
4. Howard Hughes Medical Institute, University of California San Diego, La Jolla, California 92093, USA
5. Department of Chemistry and Biochemistry, University of California San Diego, La Jolla, California 92093-0365, USA
6. Department of Pharmacology, University of California San Diego, La Jolla, California 92093-0636, USA