1. Abteilung Virologie, Institut für Medizinische Mikrobiologie und Hygiene, Universität Freiburg, D-79008 Freiburg, Germany.
2. Present address: Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305.
3. European Molecular Biology Laboratory, D-69117 Heidelberg, Germany.
4. School of Molecular and Microbial Sciences, University of Queensland, Brisbane, Australia 4072.