Currently, adult-type diffusely infiltrating gliomas are subdivided into IDH-mutant astrocytoma, IDH-mutant and 1p/19q-codeleted oligodendroglioma, and IDH-wild-type glioblastoma.1 With an incidence of 0.44 per 100 000 individuals and nearly 3000 cases annually in the United States, IDH-mutant astrocytoma comprises approximately 11% of all of these diffusely infiltrating gliomas.2 The gene discussed is IDH2; the disease is astrocytoma (excluding glioblastoma).