1. Institute of Virology, Medical Center University of Freiburg, D-79104 Freiburg, Germany;
2. Faculty of Medicine, University of Freiburg, D-79104 Freiburg, Germany
3. National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, USA
4. Institute of Medical Virology, University of Zürich, 8057 Zürich, Switzerland