Owing to this specific pattern of expression during embryogenesis, which is retained during the process of malignant transformation, OCT3/4 is an established and highly informative diagnostic marker for defined types of malignant germ cell tumours (GCTs), especially those of the seminomatous cell type (seminoma (SE), dysgerminoma and germinoma) and embryonal carcinoma (Looijenga et al, 2003; de Jong and Looijenga, 2006; Cheng et al, 2007; Looijenga, 2009). The gene discussed is POU5F1; the disease is seminoma.