1. Howard Hughes Medical Institute, Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
2. Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biology, Harvard University, Cambridge, MA 02138.
3. Agouron Pharmaceuticals, Inc., La Jolla, CA 92037.
4. Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD 21205.