Hypogonadotropic hypogonadism is often associated with anosmia in a condition known as Kallmann's syndrome. The gene for the X-linked form of Kallmann's syndrome has been mapped to chromosome Xp22.3,¹ and several mutations have been described.²–⁴ In idiopathic hypogonadotropic hypogonadism there is no anosmia, and the involved genes have not been characterized. One possible candidate is the gene for gonadotropin-releasing hormone (GnRH), especially since hypogonadal mice with the deletion of this gene have been identified.⁵ However, no abnormality of the gene for GnRH has been found in several patients with idiopathic hypogonadotropic hypogonadism.⁶–⁹ The gene for the GnRH receptor . . .