1. National Center for Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA 30333, USA.
2. Departments of Biochemistry and Biophysics, University of California–San Francisco, San Francisco, CA 94143, USA.
3. Department of Virology, Erasmus University, Rotterdam, 3000 DR, Netherlands.
4. Department of Virology, Bernhard Nocht Institute for Tropical Medicine, 20359 Hamburg, Germany.