1. P. B. Harbury is at the Whitehead Institute for Biomedical Research, Howard Hughes Medical Institute and Department of Biology, Massachusetts Institute of Technology, Nine Cambridge Center, Cambridge, MA 02142, USA, and Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, MA 02115, USA. J. J. Plecs is in the Department of Molecular and Cell Biology and Department of Physics, University of California at Berkeley, Berkeley, CA 94720, USA. B. Tidor is in the...