1. CSIRO Livestock Industries, Geelong, Victoria 3220, Australia
2. Department of Virology, University of Freiburg, D-79104 Freiburg, Germany
3. Department of Internal Medicine II, Molecular Biology, University Hospital Freiburg, 79016, Freiburg, Germany
4. Department of Molecular and Cell Biology, Unit 3044, University of Connecticut, Storrs, CT 06269