1. Department of Bioengineering, University of California, San Diego, La Jolla, CA 92093–0412, USA.
2. San Diego Center for Systems Biology, University of California, San Diego, La Jolla, CA 92093–0412, USA.
3. BioCircuits Institute, University of California, San Diego, La Jolla, CA 92093–0402, USA.
4. Molecular Biology Section, Division of Biological Sciences, University of California, San Diego, La Jolla, CA 92093–0368, USA.