GDNF and central nervous system cancer: TAMs are recruited by glioma-cell-derived factors, such as colony stimulating factor 1 (CSF-1), glial cell line-derived neurotrophic factor (GDNF), chemokine (CC motif) ligand 2 (CCL2), stromal cell-derived factor 1 (SFD-1), chemokine (CXC motif) ligand 1 (CXCL1), EGF, and granulocyte-macrophage colony-stimulating factor (GM-CSF) [96].