We thank Chihiro Takigawa, Junko Iida, Masako Shirasaka, Yumi Ohuchi, and Megumi Narita for their technical and administrative assistance. We are grateful to Drs. Minoru Ko and Azim Surani for sharing helpful data prior to publication, Dr. Hitoshi Niwa for generously providing various vectors, and Dr. Robert Farese Jr. for his kind gift of the RF8 ES cells. This work was supported in part by research grants from the Ministry of Education, Culture, Sports, Science, and Technology of Japan, the Uehara Memorial Foundation, the Naito Foundation, the Sumitomo Research Foundation, the Mitsubishi Foundation. S.Y. was supported by a Toray Science and Technology Grant (to S.Y.). This work was also supported in part by a Grant-in-Aid for 21st Century COE (Center of Excellence) Research from the Ministry of Education, Culture, Sports, Science, and Technology.