1. Laboratory of Structural Biology, National Institute of Arthritis, Musculoskeletal and Skin Diseases,1 and
2. Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 021152; and
3. Computational Bioscience and Engineering Laboratory, Center for Information Technology,3 National Institutes of Health, Bethesda, Maryland 20892;
4. Committee on Higher Degrees in Biophysics, Harvard University, Cambridge, Massachusetts 021384