Kallmann syndrome (hypogonadotropic hypogonadism with anosmia or hyposmia) results when disruption of olfactory migration also disrupts the migration of hypothalamic GnRH-producing neurons from their birthplace in the nose into the central nervous system along the olfactory/vomeronasal nerve. The gene discussed is GNRH1; the disease is hypogonadotropic hypogonadism.