OCT3/4 (OCT4, FOU5F1) is a nuclear POU-domain octamer binding transcription factor specifically expressed in cells with pluripotent capacity, i.e., embryonic stem cells and germinal cells. The gene is located on chromosome 6p21.3.
In virtually all cases of seminoma/dysgerminoma/germinoma, embryonal carcinoma, and the germ cell component of gonadoblastoma, OCT3/4 is expressed strongly in more than 90% of the nuclei, sometimes with a dot-like cytoplasmic reaction as well. Also the germinal tumour precursor lesion, intratubular germ cell neoplasia unclassified (IGCNU), is strongly stained. OCT3/4 is not found in other germ cell tumours (spermatocytic seminoma, choriocarcinoma or yolk sac tumour). Other tumour types are virtually always negative (very rarely, staining – mostly focal and weak – has been seen in carcinomas of the bladder, kidney and lung, and in Ewing sarcoma).
Classification of germ cell tumours, viz. identification of seminoma/dysgerminoma/germinoma, embryonal carcinoma, and gonadoblastoma, as well as IGCNU. OCT3/4 gives a better signal/noise ratio than placental alkaline phosphatase (PLAP) in seminoma and related tumours, and is more sensitive than CD30 in embryonal carcinoma, particularly after chemotherapy (where CD30 is frequently lost).
At present, no easily accessible normal tissue expressing OCT3/4 has been identified and GCNIS seems to be the preferred recommendable control in which the neoplastic cells must show an as strong as possible nuclear staining reaction (a minimal cytoplasmic staining reaction must be accepted). Placenta or colon/appendix can serve as negative tissue control showing no nuclear staining reaction.