As shown in Figure 1A, CXCR4 expression was detected at very low levels in MCF-10A cells in respect with ERα-positive MCF-7 breast cancer cells, while higher CXCR4 levels were observed in ER-negative MDA-MB-231 breast cancer cells, which are well-characterized in terms of their metastatic potential and properties. This evidence concerns the gene ESR1 and breast cancer.