IDH1 and oligodendroglioma: In addition to IDH1, frequently mutated genes included NOTCH1 (42.1%), ARID1A (26.3%), ARID1B (26.3%), BRCA2 (26.3%), and PIK3CA (21.0%), but mutation frequencies (Fig. 2A) and mutation number (Fig. 2B) did not differ significantly between oligodendroglioma and anaplastic oligodendroglioma.