ESR1 and breast cancer: The binding affinity of BPA with both ERs has been estimated to be 0.1–0.01 % of the affinity of 17β-estradiol (E2) [14] but data from the in vitro experiments on ER positive human MCF-7 breast cancer cells show that although it competes more effectively for binding with the ERα it induces the ERα- and ERβ-mediated gene expression with comparable efficacy [15].