1. Department of Pharmacology, Weill Medical College of Cornell University, New York, New York 10021;
2. Department of Biophysics and Biophysical Chemistry, Howard Hughes Medical Institute, Johns Hopkins University, Baltimore, Maryland 21205;
3. Department of Biochemistry, Albert Einstein College of Medicine, Yeshiva University, Bronx, New York 10465;
4. Department of Molecular Biology and Genetics, Johns Hopkins University, Baltimore, Maryland 21205;