CXCR4 and neoplasm: Nevertheless, poor prognosis could be justified in HER2-overexpressing tumors as it seems that reduced CXCL12 negative primary tumor cells are in a better position to receive endocrine CXCL12 signals, promoting their migration towards ectopic sources of the ligand through a chemotactic gradient, together with HER2 upregulation of CXCR4 tumor cells that could metastasize to distant organs.