The 2021 WHO Classification of Tumors of the Central Nervous System, 5th edition (WHO CNS5), introduced revised diagnostic criteria for pediatric brain tumors (BTs), redefining pediatric-type diffuse high-grade gliomas (pHGGs) into 4 subtypes: diffuse midline glioma, H3 K27-altered (DMG-H3K27), diffuse hemispheric glioma, H3 G34-mutant (DHG-H3G34), diffuse pediatric-type high-grade glioma, H3-wildtype and IDH-wildtype (DpHGG-H3wt/IDHwt), and infant-type hemispheric glioma (IHG). This evidence concerns the gene IDH1 and central nervous system cancer.