MTOR and glioblastoma: Platelet derived growth factor receptor (PDGFR) activity is deregulated in human GBM due to amplification and rearrangement of the PDGFR-alpha gene locus or overexpression of the PDGF ligand, resulting in the activation of downstream kinases such as phosphatidylinositol 3-kinase (PI3K), Akt, and mammalian target of rapamycin (mTOR).