1. Howard Hughes Medical Institute, University of California, Los Angeles, Los Angeles, CA 90095, USA.
2. Institute for Genomics and Proteomics, University of California, Los Angeles, Los Angeles, CA 90095, USA.
3. Department of Mathematics, University of California, Los Angeles, Los Angeles, CA 90095, USA.
4. Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, CA 90095, USA.