Inhibition of ERα activity using the antagonist tamoxifen, or drugs that inhibit estrogen biosynthesis by blocking the activity of aromatase, a key enzyme in estrogen biosynthesis (e.g. anastrozole, letrozole) [2], provide important strategies for the management of ERα-positive breast cancer and significantly reduce recurrence and breast cancer mortality. This evidence concerns the gene CYP19A1 and breast cancer.