CCL2 and glioblastoma: GBM cancer cells can attract monocyte, microglia, and macrophage by the production of several factors including CCL2 (C-C motif chemokine ligand 2), CCL7 (C-C motif chemokine ligand 7), GDNF (glial cell-derived neurotrophic factor), SDF1 (stromal cell-derived factor 1), TNF (tumor necrosis factor), VEGF (vascular endothelial growth factor), ATP (adenosine triphosphate), CSF-1 (colony-stimulating factor 1), GM-CSF (granulocyte-macrophage colony-stimulating factor), and expression of OLIG2 (oligodendrocyte transcription factor 2).