These were human breast cancer cell lines MCF-7 (ERα+/ERβ+) and MDA-MB-231 (ERα−/ERβ+) and human breast epithelial cell lines MCF-10F (ERα−/ERβ+) and MCF-10A (ERα−/ERβ−), which are well documented for their differential expression of these receptors (Fuqua et al. 1999; Girdler et al. 2001; Hu et al. 1998; Jiang and Jordan 1992; Tong et al. 2002). Here, ESR1 is linked to breast cancer.