TP53 and glioblastoma: PTEN−/−;NF1−/− iHGGs were found to express higher levels of O6-methylguanine DNA methyl transferase (MGMT) (Fig. 3e), which is associated with resistance to TMZ in GBM patients26, compared to TP53−/−;PDGFRAΔ8–9 iHGGs.