1. Graduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan; Department of Internal Medicine II, Kumamoto University School of Medicine, Kumamoto 860-8556, Japan; Yokohama Laboratory, SC BioSciences Corporation, Hodogaya-ku, Yokohama 240-0005, Japan; and Experimental Retrovirology Section, Medicine Branch, Division of Clinical Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892