ESR1 and breast cancer: To investigate whether the differential E2-regulation of CXCR4 and CXCR7 could be extended to other breast cancer cell lines, we analyzed the expression of the components of the CXCL12 axis in ER-positive ZR-75 (Fig. 2A) and ER-negative MDA-MB-231 (Fig. 2B) breast cancer cell lines.