1. R. K. Agrawal, P. Penczek, R. A. Grassucci, Y. Li, A. Leith, Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, NY 12201-0509, USA.
2. K. H. Nierhaus, Max-Planck-Institut für Molekulare Genetik, Ihnestrasse 73, D-14195 Berlin, Germany.
3. J. Frank, Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, NY 12201-0509, USA, and Department of Biomedical Sciences, State University of New York at Albany, 1400 Washington Avenue, Albany, New York 12222, USA.