Tamoxifen ((Z)-2-see figure 7) has become the treatment of choice for women diagnosed with all stages of hormone-responsive breast cancer, that is, breast cancer that is both ER and/or progesterone positive. In the US, it is also administered for prophylactic chemoprevention in women identified as high risk for breast cancer. Tamoxifen is a pure antiestrogenic trans-isomer and has differential actions at estrogen target tissues throughout the body. Tamoxifen is selectively antiestrogenic in the breast but estrogen-like in bones and endometrial cancer. Tamoxifen undergo phase I metabolism in the liver by microsomal cytochrome P450 (CYP) enzymes. The major metabolites of tamoxifen are N-desmethyltamoxifen and 4-hydroxytamoxifen.