Two key transcription factors, Oct4 and Sox2, have been identified that are crucial for maintenance of the pluripotent state of ESCs. Octamer 3/4 (Oct 3/4), a member of the POU family, is considered to be an important stem cell marker and essential transcription factor during human embryogenesis. Sox2 is a member of the Sox (SRY-Related HMG box) gene family that encodes transcription factors with a single HMG DNA-binding domain. Sox2 controls neural progenitor cells by prohibiting their ability to differentiate.  In recent years, there have also been reports about the presence of Oct 3/4 and Sox2 in differentiated benign and malignant human cells. Oct4 and Sox2 showed cooperative binding to the Sox-Oct motifs on some target promoter regions.  In addition, the transcription factor Nanog, an established regulator of pluripotency, is transcriptionally regulated directly by Oct4 and Sox2.