RB1 and glioblastoma: As further detailed below, previous studies elucidating pathway alterations in glioblastoma (GBM) have identified frequent amplifications, deletions and mutations of certain genes belonging to Phosphatidylinositol-3-kinase (PI3K)/Mitogen-activated Protein Kinase (MAPK), p53 and Retinoblastoma Protein (Rb) pathways.