1. Key Laboratory of Immunology of AIDS, Ministry of Health, the First Affiliated Hospital, China Medical University, Shenyang, Liaoning Province, P.R. China 110001.
2. Laboratoty of Molecular Virology and Epidemiology, AIDS Research Center, National Institute of Infectious Diseases, 1-23-1 Toyama, Shinjuku-ku, Tokyo 162-8640 Japan.