1. W. M. Keck Laboratory for Structural Biology, University of Maryland Institute for Bioscience and Biotechnology Research, Rockville, Maryland 20850
2. Boston Biomedical Research Institute, Watertown, Massachusetts 02472
3. Department of Biochemistry, University of Illinois, Urbana, Illinois 61801
4. Symphogen A/S, DK-2800 Lyngby, Denmark