GDNF and neoplasm: Secretion of cytokine and chemokine gradients released by GBM, like CC chemokine ligand 2 (CCL2) and CCL7, glial cell line‐derived neurotrophic factor (GDNF), hepatocyte growth factor (HGF), and so on [47, 48], induces entry of immune cells to tumor microenvironment, thus influencing glioma‐associated microglia/macrophages and fostering tumor maintenance and progression.