1. M. Rief, F. Oesterhelt, H. E. Gaub, Lehrstuhl für Angewandte Physik, Amalienstrasse 54, 80799 München, Germany.
2. M. Gautel, Biological Structures Division, European Molecular Biology Laboratory, Postfach 102209, 69012 Heidelberg, Germany.
3. J. M. Fernandez, Department of Physiology and Biophysics, Mayo Clinic, Rochester, MN 55905, USA.