In the 2021 WHO classification adult-type diffuse gliomas are classified based on both traditional histological features and molecular characteristics (isocitrate dehydrogenase [IDH1 or IDH2] mutations and 1p/19q codeletion).1 Lower-grade gliomas (ie, WHO grade 2 or 3) include IDH1- or IDH2-mutant, 1p/19q-codeleted oligodendrogliomas, and IDH1- or IDH2-mutant astrocytomas. The gene discussed is IDH1; the disease is central nervous system cancer.