The 3A2A20 antibody reacts with human OCT4 (octamer-binding transcription factor 4; also known as OCT3 and OCT3/4), an ~40 kDa homeodomain transcription factor belonging to the POU family, which is expressed in undifferentiated human embryonic stem (ES), induced pluripotent stem (iPS), embryonal carcinoma (EC) and embryonic germ (EG) cells. OCT4 binds to the octamer motif (5′-ATTTGCAT-3′) and plays a key role in maintaining cells in a pluripotent state by interacting with other transcription factors such as SOX2 to regulate the expression of several genes, including FBX15, FGF-4, REX1, SOX2 and osteopontin. Levels of OCT4 are down-regulated during differentiation and it has thus emerged as a useful marker of pluripotency in stem cells, as well as a marker for certain human malignant germ cell tumours. Expression of OCT4 together with other transcription factors has been used to reprogram somatic cells into iPS cells. Multiple isoforms of OCT4 have been observed and in humans at least two are functionally active.