1. C. L. Brooks III is in the Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
2. J. N. Onuchic is in the Department of Physics, University of California at San Diego, La Jolla, CA 92093, USA.
3. D. J. Wales is in the Department of Chemistry, Cambridge University, Lensfield Road, Cambridge CB2 1EW, UK.