The tumors from a 101-adult-patient cohort with WHO grade IV diffuse glioma were initially classified based on IHC into IDH wild-type glioblastoma (90 cases), IDH-mutant glioblastoma (eight cases), and diffuse midline glioma (DMG) with histone H3 K27M mutation (three cases) (Figure 1A). The gene discussed is IDH1; the disease is glioblastoma.