CXCR4 and cancer: The interaction between CXCR4 and CXCL12 mediates the progression and metastasis of breast and other cancers through a chemotactic effect; in the case of breast cancer, primary tumor cells overexpress CXCR4, whereas preferred organ sites of metastasis (lung, liver and bone) overexpress the CXCL12 ligand (3).