CXCR4 and cancer: Another modulator of stromal cells’ derived chemoresistance is the SDF-1α/CXCR4 axis, in which PSCs promote resistance to gemcitabine via paracrine SDF-1α/CXCR4 signaling-induced activation of FAK-AKT and ERK1/2 signaling pathways with subsequent upregulation of IL-6 in cancer cells [38].