GNRH1 and hypogonadotropic hypogonadism: The first evidence for such a reproductive dimension came from observations in Sirt1 null mice, which documented a state of hypogonadotropic hypogonadism, due to impaired migration of GnRH neurons (57), that renders Sirt1 KO mice infertile (58), due to a dramatic reduction in the number of GnRH neurons (57) and in the levels of circulating gonadotropins (59).