The discovery of IDH mutations in a subset of glioblastomas 10 years ago has fundamentally changed the approach of brain tumour diagnostics. Soon after this discovery, it emerged that low‐ and high‐grade oligodendrogliomas, diffuse and anaplastic astrocytomas, a proportion of glioblastomas and the now defunct oligoastrocytomas carry mutations in the IDH1 or IDH2 genes. Subsequently, the IDH‐mutant tumours were stratified with additional biomarkers, which has led to the definition of the WHO (2016) classes IDH‐mutant, 1p/19q codeleted (and TERT promoter mutant) oligodendroglioma and IDH‐mutant astrocytoma, with its characteristic loss of ATRX protein expression, assessed by immunostaining.