Diffusely infiltrating gliomas in adults include three main tumor types with distinct disease progression, treatment responses, and outcomes: IDH‐mutant and 1p/19q‐codeleted oligodendrogliomas have the best prognosis; IDH‐mutant astrocytomas have intermediate outcomes; and IDH‐wild‐type glioblastomas have a poor prognosis. The gene discussed is IDH1; the disease is astrocytoma (excluding glioblastoma).