Today, data on molecular genetics can outperform or complement pure histology; several tumor entities of glioblastoma, medulloblastoma, diffuse midline glioma, and ependymoma can now further be characterized by their genetic status as isocitrate dehydrogenase (IDH)-wildtype, IDH-mutant, histone H3 K27M-mutant, rearrangement during ependymal cell differentiation (RELA) fusion-positive, WNT-activated, and sonic hedgehog (SHH)-activated. The gene discussed is IDH1; the disease is neoplasm.