According to the World Health Organization Classification of Tumors of the Central Nervous System 2016 diffuse astrocytic tumor and oligodendroglial tumors are differentiated by the presence of isocitrate dehydrogenase 1 or 2 (IDH1/2) mutation and the combined loss of the short arm of chromosome 1 and the long arm of chromosome 19 (1p/19q co-deletion). Diffuse astrocytoma IDH mutant often has p53 and alpha-thalassemia/mental retardation syndrome X-linked (ATRX) mutation, showing the alternative lengthening of telomeres (ALT) phenotype, while Oligodendroglioma, IDH-mutant & 1p/19q-codeleted often have wild-type p53 and telomerase reverse transcriptase (TERT) promoter mutation, showing telomerase activation. Ohba et al. analyzed IDH, ATRX, and TERT promoter mutations, and the correlation between them. Immortalized cells overcome the telomere-related crisis by activating telomerase or ALT. In glioma, telomerase is mainly activated by TERT promoter mutation, while ALT is usually associated with ATRX mutation. Although the mechanism of how ATRX mutation induces ALT remains unclear, ATRX loss alone is believed to be insufficient to induce ALT. Treatments targeting telomere maintenance are promising 1).