Examination of endocrine mechanisms accounting for the masculinized external genitalia of male and female spotted hyenas has revealed developmental events that are androgen-independent, androgen-dependent, and surprisingly developmental processes that appear to be estrogen-dependent. Further examination of "estrogen mutant" mice (αERKO, ERα null, AROM+, and aromatase knockout) reveals profound masculinization in all females and more subtle effects in males. The results suggest an important role of estrogen in development of the external genitalia.