The receptors for follitropin (FSHR), thyrotropin (TSHR), and lutropin/chorionic gonadotropin (LHCGR) are the members of the glycoprotein hormone (GPH) receptors (GPHR) family. They present a bipartite structure with a large extracellular amino-terminal domain (ECD), responsible for high-affinity hormone binding, and a carboxyl-terminal serpentine region, implicated in transduction of the activation signal. Spontaneous ovarian hyperstimulation syndrome (sOHSS) is a rare genetic condition in which human chorionic gonadotropin (hCG) promiscuously stimulates the FSHR during the first trimester of pregnancy. Surprisingly, germline FSHR mutations responsible for the disease have so far been found only in the transmembrane helices of the serpentine region of the FSHR, outside the hormone binding domain. When tested functionally, all mutants were abnormally sensitive to both hCG and thyrotropin (TSH) while displaying constitutive activity. This loss of ligand specificity was attributed to the lowering of an intramolecular barrier of activation rather than to an increase of binding affinity. Here we report the first germline mutation responsible for sOHSS (c.383C > A, p.Ser128Tyr), located in the ECD of the FSHR. Contrary to the mutations described previously, the p.Ser128Tyr FSHR mutant displayed increase in affinity and sensitivity toward hCG and did not show any constitutive activity, nor promiscuous activation by TSH. Thus, sOHSS can be achieved from different molecular mechanisms involving each functional domains of the FSHR. Based on the structure of the FSHR/FSH complex and site-directed mutagenesis studies, we provide robust molecular models for the GPH/GPHR complexes and we propose a molecular explanation to the binding characteristics of the p.Ser128Tyr mutant. © 2007 Wiley-Liss, Inc.