1. Swiss Institute for Experimental Cancer Research and the Institute of Biochemistry, University of Lausanne, CH-1066 Epalinges, Switzerland1;
2. Laboratoire de Microbiologie INRA, 63122 St. Genes-Champanelle,2 and
3. Department of Virology, Smyttskyddsinstitutet, Karolinska Institute, 105 21 Stockholm, Sweden3
4. Laboratoire de Virologie et d'Immunologie Moléculaire INRA, C.R.J. Domaine de Vilvert, 78350 Jouy-en-Josas,4 France; and