Tamoxifen is a selective estrogen receptor modulator (SERM) widely used in breast cancer treatment and prevention that has seldom been studied for treatment of benign diseases of the breast, but a decrease in risk of developing FA [22], and a size reduction of existing FA [23] have been shown to be induced by tamoxifen; however the rate of shrinkage has not been explored. The gene discussed is ESR1; the disease is breast fibrocystic disease.