To address the role of TGF-β at different stages of breast cancer progression, we have used four MCF10A-derived cell lines developed by Miller and co-workers [37,38], as schematized in Figure 1A. Although the parental nontumorigenic MCF10A cell line is ER negative, the derivative cell lines show varying degrees of ER positivity and biological responses to estrogen in vitro and in vivo, and have other characteristics of ER+ breast cancer (see Methods). This evidence concerns the gene ESR1 and breast cancer.