The CXCR4-CXCL12 axis is necessary for mediating the attraction of monocytic MDSCs into the tumor environment and PGE2 is important to regulation of CXCL12 production in cancer-associated fibroblasts and the cancer environment, and to enhancement of CXCR4 expression in Mo-MDSCs [29]. Here, CXCL12 is linked to cancer.