Three clinically relevant subgroups of diffuse low-grade (grade II) gliomas are identified and recognized by the WHO 2016 classification based on two molecular markers: (1) oligodendroglioma, IDH mutant and 1p/19q-codeleted (IDH1/2 mutation in combination with presence of a co-deletion of the entire 1p and 19q chromosomal arms), (2) diffuse astrocytoma, IDH mutant; (IDH1/2 mutation without 1p19q co-deletion), and (3) diffuse astrocytoma, IDH1/2 wildtype. The gene discussed is IDH1; the disease is diffuse astrocytoma.