CXCL12 and central nervous system cancer: Microglia and glioma cells secrete stromal-derived factor-1 (SDF-1 or CXCL12), a microglia recruitment molecule that normally binds to its receptor CXCR4, a specific receptor for CXCL12 expressed on microglia, to form the CXCR4-CXCL12 axis that attracts microglia to hypoxic regions of the tumor and mediates tumor proliferation, abnormal angiogenesis, invasion and drug resistance (110–112).