TAC3 and hypogonadotropic hypogonadism: (3, 11)] have been involved in the regulation of the gonadotropic axis, after the discovery that mutations of tac3 or tacr3 genes in humans resulted in a hypogonadotropic hypogonadism (12, 13), reversible in adulthood (14) Similarly, although fertile, tac3 or tacr3 null mice exhibited central reproductive defects such as an abnormal estrous cyclicity (15, 16).