Indeed, three randomized clinical trials have compared aromatase inhibitor versus tamoxifen treatment effectiveness in adjuvant breast cancer settings for all BMI categories: the ATAC (Arimidex, Tamoxifen Alone or in Combination) and the BIG (Breast International Group) 1–98 trials conducted in postmenopausal women, and the ABCSG (Austrian Breast and Colorectal Cancer Study Group) 12 trial in premenopausal ones. This evidence concerns the gene CYP19A1 and breast cancer.