POLE and glioblastoma: After 5 days of incubation with 0.5 and 5.0 μM 6-TG, the intensity of crystal violet staining was significantly increased in POLE KO compared to WT LN-229 glioblastoma cells (two-tailed Student’s t test, p = 0.040 and p = 0.044), indicating more cells with 6-TG resistance, i.e. a higher HPRT1 mutation rate, among POLE KO cells (Fig. 5f, g).