CCL21 and breast carcinoma: As an example, it has been shown that the interaction between the chemokine receptors CXCR4 and CCR7, overexpressed on breast cancer and melanoma cells, and their relative ligands (CXCL12/SDF-1α and CCL21/6Ckine), in turn overproduced on the endothelium of some of the main target organs such as lung, liver, lymph nodes, and bone marrow, participate in the metastatic process [4].