1. Fred Hutchinson Cancer Research Center, Program in Structural Biology, Division of Basic Sciences, 1 124 Columbia Street A3-023 Seattle, WA 98104.
2. Molecular Research Council, Laboratory of Molecular Biology, Hills Road, Cambridge England CB2 2QH.
3. Brookhaven National Laboratories, Department of Structural Biology, Upton, Long Island, NY 11973.
4. Brookhaven National Laboratories, Department of Biology, Upton, Long Island, NY 11973.
5. Department of Molecular and Cell Biology, Stanley Hall, University of California, Berkeley, CA 94720.