1. Laboratory of Molecular and Cellular Biophysics, National Institute of Child Health and Human Development, Bethesda, Maryland 20892
2. Unit of Human Virology, DIBIT San Raffaele Scientific Institute, Milano, Italy
3. Rega Institute for Medical Research, Katholieke Universiteit Leuven, Leuven, Belgium
4. Department of Medical Sciences, University of Cagliari Medical School, Cagliari, Italy