IDH1 and oligodendroglioma: In the 5th edition of the WHO Classification of Central Nervous System Tumors, released in 2021, adult-type diffuse gliomas were classified with a combination of histologic and molecular features as oligodendrogliomas (~ 7% of diffuse gliomas) in the presence of mutation in either IDH1 or IDH2 (hereafter grouped simply as “IDH mutation”) and co-deletion of chromosomal arms 1p and 19q, IDH-mutant astrocytoma (~ 11%) in the presence of an IDH mutation and retained 1p/19q, and IDH-wild-type glioblastoma (~ 82%) in the absence of either of these molecular alterations [127, 163].