When ERα was ectopically expressed in an ER-negative breast cancer cell line (MDA-MB231), 17-β-oestradiol inhibited the proliferation of these cells, whereas the anti-oestrogens ICI182780 and tamoxifen blocked this effect indicating that ERα re-expression restores tamoxifen sensitivity in ER-negative cells [138]. This evidence concerns the gene ESR1 and breast cancer.