GBM remained classified as a grade IV tumor, corresponding to the highest degree of malignancy, and was subclassified into the following 3 categories: isocitrate dehydrogenase (IDH)-wildtype, IDH-mutant, and not otherwise specified (NOS).4 IDH-wildtype is the most common variant of this disease, accounting for 90% of GBM cases. Here, IDH1 is linked to glioblastoma.