IDH1 and astrocytoma (excluding glioblastoma): Oligodendrogliomas (WHO grades 2–3) are now defined by the presence of simultaneous deletion of chromosomal arms 1p and 19q, as well as mutation in either IDH1 or IDH2. Moreover, what was previously classified by histology as “astrocytoma” and “glioblastoma” has now been divided into IDH-mutant astrocytoma (WHO grades 2–4) and IDH-wildtype glioblastoma (WHO grade 4).