IDH1 and central nervous system cancer: IDH1-IDH2 sequencing is mandatory in the case of lack of immunopositivity, in order to rule out the presence of non-canonical mutations in all glioblastomas in patients younger than 55 years, in those with loss of ATRX expression, in those with a previous history of a lower-grade glioma, and in all grade 2 or 3 diffuse gliomas.