According to the 2016 WHO classification of tumors of the central nervous system, testing for mutations in the IDH1 and IDH2 genes is recommended for all patients with grade II–III gliomas and GBM diagnosed before the age of 55. The most common mutation of IDH1 is R132H, which can be detected by immunohistochemistry (IHC). The remaining mutations of the IDH1 and IDH2 genes can be detected by DNA sequencing.