TAC3 and hypogonadotropic hypogonadism: Mutations in the genes encoding NKB (tac3) or its receptor NK3R (tacr3) lead to hypogonadotropic hypogonadism—a disease characterized by the failure of sexual maturation, impaired gametogenesis, and infertility, indicating that the NKB/NK3R system is indispensable for human reproduction (18, 19).