RB1 and glioblastoma: Additionally, primary GBMs, representing about 90% of all GBM subsets, show other less-common mutations and/or deletions in genes such as TP53, PDGFRA, EGFRvIII, PIK3CA, PIK3R1, RB1, H3F3A, MET, CDK4, CDK6, and MDM4 (24–26).