IDH1 and glioblastoma: For example, among the four GBM subtypes in Phillips's classification,21 the proneural subtype was originally considered the subtype with the longest OS, but this result was later attributed to a small subset of patients with an isocitrate dehydrogenase 1 (IDH1) mutation.22 In addition, IDH1 wild‐type proneural tumours had the worst prognosis among all GBM subtypes in the placebo arm.