CXCR4 and neoplasm: Moreover, hypoxic conditions caused by chemotherapy lead on the one hand to increased CXCR4 expression on escaped chemoresistant tumor cells and consequently to their enrichment and on the other hand to increased CXCL12 expression by tumor-associated stromal cells, further reinforcing the protumorigenic CXCL12-CXCR4 axis [19].