The diagnostic importance of IDH mutational status in diffuse gliomas was first formally recognized within the updated 4th edition of the WHO Classification of Tumours of the Central Nervous System (2016). Its introduction as a diagnostic marker was based on evidence that incorporation of biomarkers into an integrated diagnosis provided a more reproducible and clinically meaningful classification of diffuse gliomas in adults [20–22]. For IDH-mutant diffuse astrocytic gliomas, the integrated diagnostic entities (and corresponding grades) of the 2016 WHO Classification included diffuse astrocytoma,