CXCR4 and breast cancer: Importantly, as observed in a rat mammary adenocarcinoma cell line overexpressing both receptors [167], CXCR4 and CXCR7 play opposing roles in breast cancer metastasis: CXCR4 mediates cancer cell invasion allowing cells to follow the CXCL12 gradient generated by metastatic targets whereas CXCR7 favors tumor growth increasing angiogenesis but impairs cell migration scavenging the chemokine.