Indeed, SmSak is more expressed in female than in male reproductive organs whereas mouse Sak [11] and human Plk4 [13] were detected at the highest level in testes but not in ovaries, and that mutations in Plk4 caused male hypogonadism and germ cell loss in mice, suggesting a role for Plk4 in the initiation of spermatogenesis [59]. Here, PLK4 is linked to hypogonadism.