Estrogen effects on the breast may be the result of either circulating estradiol levels or locally produced estrogens. Aromatase P450 catalyzes the conversion of the C19 steroids, androstenedione, testosterone, and 16-a-hydroxyandrostenedione to estrone, estradiol-17 and estriol. As such, an overabundance of substrate or an increase in enzyme activity can increase estrogen concentrations and thus initiate the cascade to breast development in females and males. For example, in the more complete forms of androgen insensitivity syndromes in genetically male (XY) patients, excess androgen aromatizes into estrogen resulting in not only gynecomastia, but also a phenotypic female appearance. Furthermore, the biologic effects of over expression of the aromatase enzyme in female and male mice transgenic for the aromatase gene result in increased breast proliferation. In female transgenetics, over expression of aromatase promotes the induction of hyperplastic and dysplastic changes in breast tissue. Over expression of aromatase in male transgenics caused increased mammary growth and histological changes similar to gynecomastia, an increase in estrogen and progesterone receptors and an increase in downstream growth factors such as TGF-beta and bFGF (20). Interestingly, treatment with an aromatase inhibitor leads to involution of the mammalian gland phenotype (30). Thus, although androgens do not stimulate breast development directly, they may do so if they aromatize to estrogen. This occurs in cases of androgen excess or in patients with increased aromatase activity.