CYP19A1 and breast cancer: In addition to local estrogen metabolism via STS and aromatase, serum estrogen levels for E1, E1S, E2, and E2 sulfate (E2S) have been reported to fall after surgical removal of STS-positive breast cancer in postmenopausal women, implying an additional systemic effect and indicative of the importance of STS activity in forming active estrogens (17, 277).