1. H. Yin and J. Gelles, Department of Biochemistry, Biophysics Program, and Center for Complex Systems, Brandeis University, Waltham, MA 02254, USA.
2. M. D. Wang and S. M. Block, Department of Molecular Biology and Princeton Materials Institute, Princeton University, Princeton, NJ 08544, USA.
3. K. Svoboda, Biological Computation Department, AT&T Bell Laboratories, Murray Hill, NJ 07974, USA.
4. R. Landick, Department of Bacteriology, University of Wisconsin, Madison, WI 53706, USA.