Adult diffuse gliomas are now categorized into three classes: IDH1- and IDH2-mutated oligodendrogliomas, which exhibit a 1p/19q codeletion (loss of the p arm of chromosome 1 and the q arm of chromosome 19) and are associated with longer survival, representing low-grade gliomas; IDH1- and IDH2-mutated diffuse astrocytomas, which have a variable prognosis, ranging from low-grade to high-grade gliomas; and glioblastomas, which lack IDH1 or IDH2 mutations but exhibit an activating mutation in the TERT promoter (a telomere-building protein), EGFR amplification, and specific karyotypic features. The gene discussed is IDH1; the disease is glioblastoma.