Compared with the most common rs13347CC genotype, variant genotypes (CT and TT) increased an individual's susceptibility to breast cancer, especially in estrogen receptor (ER) negative patients (odds ratio (OR) = 1.37, 95%CI = 1.17 to 1.59 for ER positive patients; OR = 2.37, 95% CI = 2.00 to 2.80 for ER negative patients). Here, ESR1 is linked to breast cancer.