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). Here, STS is linked to breast cancer.