1. Chemical Biology Laboratory, Molecular Discovery Program, Center for Cancer Research, National Cancer Institute, National Institutes of Health, NCI—Frederick, Frederick, Maryland 21702, United States
2. Macromolecular Crystallography Laboratory, Center for Cancer Research, National Cancer Institute, National Institutes of Health, NCI—Frederick, Frederick, Maryland 21702, United States
3. Laboratory of Molecular Immunology, United States Army Medical Research Institute of Infectious Diseases, Frederick, Maryland 21702, United States