Finally, we also examined Sox9 expression in several ER-positive (MCF7, T47D and ZR75-1) and ER-negative (MDA-MB-231 and MDA-MB-468) breast cancer cell lines and found that, as in breast tumour samples, Sox9 is more highly expressed in cell lines lacking ER than in ER-positive cells, both at the mRNA and protein levels (Fig. 3e). This evidence concerns the gene ESR1 and breast cancer.