While sporadic MMR mutations are relatively rare in diffuse gliomas, those occurring in recurrent gliomas in the setting of TMZ administration are much more frequent [17, 27, 32, 38, 179, 209], and appear to be more common for IDH-mutant astrocytomas (47%) than IDH-wild-type glioblastomas (16%) or oligodendrogliomas (25%) [17, 94]. Here, IDH1 is linked to oligodendroglioma.