Studies of genetic males with single gene mutations that impair testosterone formation or action and consequently prevent development of the normal male phenotype provide unique insight into the control of gender role behavior. 46,XY individuals with either of two autosomal recessive mutations [17β-hydroxysteroid dehydrogenase 3 (17β-HSD3) deficiency or steroid 5α-reductase 2 (5α-R2) deficiency] have a female phenotype at birth and are raised as females but frequently change gender role behavior to male after the expected time of puberty. In contrast, genetic males with mutations that impair profoundly the function of the androgen receptor are also raised as females and have consistent female behavior as adults. Furthermore, the rare men with mutations that impair estrogen synthesis or the estrogen receptor have male gender role behavior. These findings indicate that androgens are important determinants of gender role behavior (and probably of gender identity) and that this action is mediated by the androgen receptor and not the result of conversion of androgen to estrogen. The fact that all genetic males with 17β-HSD3 or 5α-R2 deficiency do not change gender role behavior indicates that other factors are also important determinants of this process.