1. The Burroughs Wellcome Fund, Research Triangle Park, NC 27709
2. Department of Molecular Biology and Pharmacology, Washington University School of Medicine, St. Louis, MO 63110
3. Department of Biophysics, Johns Hopkins University, Baltimore, MD 21218
4. Department of Molecular Biology, the Scripps Research Institute, La Jolla, CA 92037
5. Department of Molecular Genetics and Cell Biology, and
6. Department of Chemistry, University of Chicago, Chicago, IL 60637
7. Division of Applied Mathematics, Brown University, Providence, RI 02912
8. Department of Physics, University of California-San Diego, La Jolla, CA 92093
9. Department of Mathematics, Florida State University, Tallahassee, FL 32306; and
10. Department of Mathematics and Statistics, Boston University, Boston, MA 02215, USA.