Tamoxifen, a selective estrogen receptor modulator, was first approved by the U.S. Food and Drug Administration (FDA) in 1977 for the treatment of metastatic breast cancer [3], and has also been approved for primary prevention in women at high risk of breast cancer, for adjuvant treatment (given after primary treatment), and for ductal carcinoma in situ [4]. This evidence concerns the gene ESR1 and breast cancer.