Transcriptional factors, OCT3/4 (POU5F1) and STAT3 function as key regulators in maintaining pluripotency of stem cells. Thus, POU5F1 and STAT3 have been widely used as molecular markers of pluripotential stem cells. Pluripotential cell-specific Nanog gene is a newly identified homeodomain-bearing transcriptional factor. Importantly, Nanog is expressed specific to early embryos and pluripotential stem cells including mouse and human embryonic stem (ES) and embryonic germ (EG) cells. It is a key molecule involved in the signaling pathway for maintaining the capacity for self-renewal and pluripotency, bypassing regulation by the STAT3 pathway. Therefore, Nanog is one of the molecular markers suitable for recognizing the undifferentiated state of stem cells in the mouse and human.