A novel heterozygous mutation A340T leading to the substitution of Phe for the conserved amino acid Ile114 was identified by nucleotide sequencing of the human LH/chorionic gonadotropin receptor (hLHR) of a patient with Leydig cell hypoplasia. This mutation is located in the third leucine-rich repeat in the ectodomain of the hLHR. In vitro expression studies demonstrated that this mutation results in reduced ligand binding and signal transduction of the receptor. Studies of hLHR constructs in which various amino acids were substituted for the conserved Ile114 showed that receptor activity is sensitive to changes in size, shape, and charge of the side chain. A homology model of the wild-type hLHR ectodomain was made, illustrating the packing of conserved hydrophobic side chains in the protein core. Substitution of Ile114 by Phe might disrupt intermolecular contacts between hormone and receptor. This mutation might also affect an LHR-dimer interaction. Thus, the I114F mutation reduces ligand binding and signal transduction by the hLHR, and it is partially responsible for Leydig cell hypoplasia in the patient.