For instance, due to reduced oxygen supply, the central regions of GBM tumors show high levels of hypoxia and in this hypoxic region, hypoxia-inducible chemokines that attract monocytes/macrophages, such as VEGF-A, SDF-1 are enriched compared to the peritumoral region [28,65,66,67]. Here, CXCL12 is linked to glioblastoma.