Many studies have evaluated the association between SNPs in estrogen-related genes, such as CYP17, CYP19, CYP1A1, CYP1B1, COMT, GSTP1, and ERα/β, and the risk of breast cancer, especially in postmenopausal women [14,15,16,17,18,19,20,21]. Here, ESR1 is linked to breast cancer.