CXCL12 and neoplasm: Once inside the brain parenchyma, tumor cells encounter a rich microenvironment of cytokines and growth factors, predominantly produced by astrocytes (i.e., SDF-1α [73], IL-1, IL-3, IL-6, interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α), TGF-β, and PDGF-1 [74]), which the tumor cells usurp to promote survival, growth, and potentially organ-specific metastasis [62].