IDH1 and astrocytoma (excluding glioblastoma): Isocitrate dehydrogenase (IDH)-mutant astrocytomas are recognized as a single molecular entity spanning CNS WHO Grades 2-4, and clinical behavior is shaped by early lineage-defining alterations (IDH1/2, ATRX, TP53) and by later events linked to malignant transformation (e.g., CDKN2A/B homozygous deletion).