Trials in the adjuvant setting have also indicated greater efficacy for anastrozole and other aromatase inhibitors versus tamoxifen.17, 18 Additionally, the reduction in contralateral breast cancer is consistent with the benefits of anastrozole compared with tamoxifen seen in the ATAC trial17 and compared with placebo in the IBIS-II breast cancer prevention trial.11 This evidence concerns the gene CYP19A1 and breast cancer.