CCL2 and glioblastoma: In GBM, microglia and macrophages are the largest population of tumor-infiltrating cells (5–30% of tumor mass) and are actively recruited by the tumor via the secretion of chemo-attractants including monocyte chemoattractant protein (MCP-1, also known as CCL2), stromal-derived factor-1 (SDF-1) and macrophage-colony stimulating factor (M-CSF) [2].