Diffuse gliomas are characterized in diagnostic pathology by a growth pattern of individual tumor cells growing through the brain parenchyma, as opposed to the sharp pushing border of circumscribed astrocytic tumors or brain metastasis.2The morphological pattern can disclose high-grade features (such as necrosis, microvascular proliferation, and/or mitotic figures) or suggest possible molecular changes (as in adult-type oligodendroglioma, defined by IDH mutations associated with 1p/19q co-deletion) (Figure 1). The gene discussed is IDH2; the disease is oligodendroglioma.