1. W. G. Scott and A. Klug, MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, England.
2. J. B. Murray and J.R.P. Arnold, Department of Biology, University of Leeds, Leeds LS2 9JT, England.
3. B. L. Stoddard, Fred Hutchinson Cancer Research Center, 1124 Columbia Street, Seattle, WA 98104, USA.