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Fig. 1-11. Both the the beta subunits of the glycoprotein hormones and the glycoprotein hormone receptors are encoded by paralogous genes. A: Similarity of the aminoacid sequences of the  -subunits of human chrorionic gonadotropin (hCG), luteinizing hormone (LH), thyrotropin (TSH) and follicle stimulating hormone (FSH). Inset: Diagram of the general structure of the receptors for these hormones, showing the ectodomain (extracellular), serpentine (transmembrane) and endodomain (intracellular domain). B: Similarity of the aminoacid sequences of the LH/CG, FSH and TSH receptors (r). Sequence identities are indicated, separately for the extracellular domains of the three receptors. The pattern of shared similarities suggests co-evolution of the hormones and the extracellular domain of their receptors, resulting in generation of specificity barriers. The high similarity of the serpentine domains of the receptors is compatible with a conserved mechanism of intramolecular signal transduction. (Reproduced from Vassart G, Pardo L, Costagliola S. Molecular dissection of the glycoprotein hormone receptors. Trends Biochem Sci 2004;29:119, with permission of the publisher)