Embryonic stem (ES) cells were first derived from the blastocysts of mice in 1981 [1,2] and humans in 1998 [3]. ES cells have two important properties: theability to maintain pluripotency, which is the ability to differentiate into a wide variety of cells, and rapid proliferation. These characteristics make mouse ES cells an essential component of gene targeting technology. These qualitiesalso make human ES cells attractive sources for cell transplantation therapy to treat various diseases, including spinal cord injuries and juvenile diabetes. The molecular mechanisms underlying the pluripotency and rapid proliferation of ES cells are currently a major focus of the field of stem cell biology [4-6].