CYP19A1 and breast cancer: Transforming growth factor-beta (TGF-β) synthesis is upregulated in the presence of tamoxifen, which reduces mammary epithelial proliferation.37 In addition, tamoxifen causes decreased circulation of breast cancer mitogens.38 It is the hormonal agent of choice for the adjuvant treatment of premenopausal women with early-stage breast cancer, as well as postmenopausal women who are not candidates for aromatase inhibitors (AIs).