IDH-mutant diffuse astrocytomas are defined by the 2021 WHO CNS5 classification as tumors harboring a gain-of-function mutation in IDH1 or IDH2 and are graded from 2 to 4 based on histopathologic features, including anaplasia, mitotic activity, necrosis, microvascular proliferation, and homozygous CDKN2A/B deletion (1). The gene discussed is IDH1; the disease is diffuse astrocytoma.