Approximately 70% of IDH-mutant astrocytomas are known to harbor a methylated O6-methylguanine-DNA-methyltransferase (MGMT) promoter and respond to TMZ treatment, whereas almost all H3K27M-mutant diffuse midline gliomas have unmethylated MGMT promoters and generally are not sensitive to TMZ treatment. The gene discussed is IDH2; the disease is diffuse midline glioma.