Classifying IDH mutant tumors into grades 2 and 3 in oligodendrogliomas and 2, 3, and 4 in astrocytomas continues to rely on histologic appearance, except when astrocytic tumors carry homozygous deletion of CDKN2A/B making them grade 4 tumors independent of histologic characteristics. Here, IDH1 is linked to astrocytoma (excluding glioblastoma).