CXCR4 and astrocytoma (excluding glioblastoma): The CXCL12/CXCR4 axis has gained increased focus during the recent decade [107] and has been extensively studied in brain tumors (GBM, astrocytoma, medulloblastoma, oligodendroglioma, and oligodendroastrocytoma) [108] due to its key role in the communication of tumor cells with their microenvironment [109].