IDH1 and oligodendroglial tumor: For this reason, the WHO classification includes GBM as part of the diffuse astrocytic and oligodendroglial tumors group and they are divided into three subgroups based on IDH mutations: (1) glioblastoma, IDH-wildtype, clinically identified as primary GBM and predominant in patients over 55 years of age, (2) glioblastoma, IDH-mutant, clinically identified as secondary GBM and more common in younger patients, and (3) glioblastoma NOS (not otherwise specified), which does not fit into the other categories and is not well defined [9].