A considerable number of infiltrating TAMs in the GSC niche, including the blood-derived macrophages and resident microglia (198), indicates a pivotal player in GBM tumor progression and GSC maintenance by a pleiotrophin–PTPRZ1 signaling axis (199), and is also positively linked to the poor prognosis of GBM and the high malignancy grade of glioma (200, 201). Here, PTN is linked to glioblastoma.