To explore the characteristics of the methylation changes that were possibly associated with malignant transformation of IDH-mutant astrocytic tumors, comparative analysis was performed between typical G-CIMP astrocytic tumors without the codel in the C.1 cluster (C.1 non-codel gliomas; n = 44) and G-CIMP-demethylated tumors (C.3 gliomas; n = 9). The gene discussed is IDH1; the disease is glioma.