ACTN3 and hypogonadism: Testicular feminized mice (Tfm; which have an inactivating mutation in AR) and hypogonadal mice (hpg; which lack gonadotropin and sex steroid production) are both androgen deficient and both show significantly reduced Actn3 expression in the testis, while testosterone treatment of hpg mice up-regulated Actn3, confirming that Actn3 is an androgen-regulated target gene in the testis (22).