IDH1 and oligodendroglioma: Diffuse gliomas are the most common primary CNS malignancy to affect the adult population, and the three entities that constitute adult diffuse gliomas include the following: (1) astrocytoma, IDH-mutant; (2) oligodendroglioma, IDH-mutant and 1p/19q-codeleted; and (3) glioblastoma, IDH-wildtype.