CXCR4 and breast cancer: Previously, it was showed that CXCR4 is involved in homing of breast cancer cells predominantly to bone and liver [30, 31], however, this is a first study that revealed CXCR4 overexpression in the PB enriched for epithelial CTCs, and first study that showed clear association between plasma level of CXCR4 ligand SDF-1 and subset of CTCs with epithelial phenotype.