The presence of a distinct oligodendrocyte-like component of the tumor should point to either oligodendroglioma (Figure 18) - GFAP and (oligodendrocyte transcription factor) OLIG2 positive or oligoastrocytoma - OLIG2 expression in the oligodendrocytic component and diffuse GFAP expression, stronger in the astrocytic component. The gene discussed is GFAP; the disease is oligoastrocytoma.