Aaron Bensimon is head of the DNA Biophysics Laboratory. In 1992, he earned a Ph.D. in molecular biology at the Weizmann Institute of Science in Rehovot, Israel. Address for Bensimon: Laboratoire de Biophysique de l’ADN, Departement des Biotechnologies, Institut Pasteur, 25 rue du Dr. Roux, 75724 Paris Cedex 15, France. Internet: abensim@pasteur.fr