The isocitrate dehydrogenase 1/2 (IDH) mutation is the main distinguishing feature of lower-grade gliomas (LGGs, grade 2 or 3 astrocytomas/oligodendrogliomas) and secondary glioblastomas that developed from LGGs [according to the 2016 World Health Organization (WHO) classification]1. The gene discussed is IDH1; the disease is glioblastoma.