1. Department of Molecular Biology and Genetics, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
2. Laboratory of Biochemistry and Molecular Biology, The Rockefeller University, New York, NY 10021.
3. Departnent of Medicine, Johns Hopkins University School of Medicine, Baltimore, MD 21205.
4. Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10021.