CYP19A1 and breast cancer: However, in breast carcinoma tissue, aromatase (which converts androgens into estrogens), sulfatase (which hydrolyzes the estrone sulfates to estrone) and 17β-HSD1 (which converts the estrone to the potent 17β-estradiol) tend to be overexpressed, whereas the expression of estrogen sulfotransferase is frequently decreased, which may result in the accumulation of 17β-estradiol in breast cancer tissues (Fig. 2).