Human chorionic gonadotropin (hCG) is a glycoprotein hormone produced by trophoblastic cells of the placenta beginning 10 to 12 days after conception. Maintenance of the fetus in the first trimester of pregnancy requires the production of hCG, which binds to the corpus luteum of the ovary which is stimulated to produce progesterone which in turn maintains the secretory endometrium. hCG is present only in trace amounts in non pregnant urine and sera. It rises sharply during pregnancy. HCG is composed of two non identical, non covalently linked polypeptide chains designated as the a and b subunits. The a subunit of HCG is nearly identical to that of thyroid stimulating hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH). A germ cell tumor which is positive for cytokeratin, placental alkaline phosphatase (PLAP) and HCG but negative for EMA and AFP is probably a choriocarcinoma.