Although discovered in the early 1930s, much of the biochemistry and molecular biology of human chorionic gonadotrophin (hCG) has only recently been revealed. The sequence was described in 1973 (Bellisario et al. 1973; Carlsen et al. 1973), but the precise secondary and tertiary structures are still unknown. Only in the last 5 years has there been dramatic progress in the understanding of the molecular nature of this hormone. These studies shed light on many clinical aspects of the biology of hCG, including its association with non-trophoblastic epithelial cancers. CG and the glycoprotein hormones Chorionic gonadotrophin is a member of a group of four structurally homologous proteins commonly referred to as the glycoprotein hormones. As the name implies, the hormone protein chains are glycosylated; hCG contains approximately 30% carbohydrate by weight. The other members are luteinizing hormone (LH), follicle-stimulating hormone (FSH) and thyroid-stimulating hormone (TSH). CG differs from the other glycoprotein