Human Chorionic Gonadotropin (HCG)
HCG is a glycoprotein hormone secreted in pregnancy by embryonic trophoblast cells during the first 7 weeks of gestation. Production subsequently shifts to the placenta, where it continues until term. HCG is a heterodimer consisting of an alpha chain and a beta chain. The alpha chain is common to several other hormones, including lutenizing hormone (LH), follicle stimulating hormone (FSH), and thyroid stimulating hormone (TSH), while the beta chain is unique to HCG. Both alpha and beta chains contain a cysteine knot motif, also found in growth factors such as TGF-beta family members.