Mutations in IDH1 or its homolog 2 (IDH2) have been identified as early molecular events in the development of astrocytomas and oligodendrogliomas, and they occur in various types of malignancies, including IDH-mutant glioblastoma (grade IV) (Figure 2A) [7,40,41]. The gene discussed is IDH1; the disease is astrocytoma (excluding glioblastoma).