Octamer-4 (Oct-4): Oct-4, encoded by the gene POU5F1, is a transcription factor that is highly expressed in undifferentiated embryonic stem cells compared to other somatic cells. Oct-4 expression in embryonic stem cells is critical to maintain pluripotency. In fact, when Oct-4 experession is experimentally knocked out, ES cells spontaneously differentiate.

In 2006, the Yamanaka lab identified Oct-4 as one of the four factors that, when co-transfected and expressed in mouse adult fibroblasts, caused fibroblasts to revert to an embryonic-like state. One year later, the same four factors where used to successfully reprogram human adult fibroblast cells into induced pluripotent stem cells. These four factors are Oct-4, SOX2, Klf-4 and c-Myc.