1. Unité de Biologie des Rétrovirus, Institut Pasteur, 75724 Paris Cedex 15,1 and
2. Institute for Virus Research, Kyoto University, Kawaharacho, Shogoin, Sakyo, Kyoto 606, Japan2; and
3. Unité INSERM U479, Hopital Bichat, 75018 Paris,3 France;
4. Infectious Diseases and Immunology, Mayo Clinic, Rochester, Minnesota 559054