CYP19A1 and breast cancer: Biologically active estrogens are synthesized by estrogen-producing enzymes, such as aromatase, which converts circulating androstenedione to estrone or testosterone to estradiol; steroid sulfatase (STS), which hydrolyzes of circulating estrone sulfate to estrone; and 17β-hydroxysteroid dehydrogenase type 1 (17β-HSD1), which converts estrone to estradiol in breast cancer (Figure 2) [16].