1. Department of Biochemistry and Molecular Biology, University of Barcelona, Martí i Franquès 1, 08028 Barcelona, Spain
2. Division of Tumor Biology, Dana Farber Cancer Institute, Harvard Medical School, Boston, MA 02115, U.S.A.
3. The AIDS Research Center, National Institute of Infectious Diseases, Tokyo 162-0052, Japan
4. Department of Cellular Immunology, Institut Pasteur, 28 rue du Docteur Roux, 75724 Paris cedex 15, Paris, France