1. H. Shibuya, Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060, and Precursory Research for Embryonic Science and Technology, Research Development Corporation of Japan, Hikari-dai, Kyoto 619-02, Japan.
2. K. Yamaguchi, K. Irie, K. Matsumoto, Department of Molecular Biology, Faculty of Science, Nagoya University, Chikusa-ku, Nagoya 464-01, Japan.
3. K. Shirakabe, Y. Gotoh, E. Nishida, Department of Genetics and Molecular Biology, Institute for Virus Research, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan.
4. A. Tonegawa and N. Ueno, Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060, Japan.