A histology panel review of 455 IDH mutant astrocytomas from 2 European Organization for Research and Treatment of Cancer (EORTC) clinical trials identified the mitotic index, using a cutoff of 2 mitoses per 10 high-power fields, as an independent prognostic marker in tumors lacking homozygous CDKN2A/B deletion. Here, IDH1 is linked to astrocytoma (excluding glioblastoma).