No difference was noted in 2021 WHO grade between MMR-mutant and MMR-wild-type IDH-mutant astrocytomas at either the initial tumor presentation or recurrence; however, 67% of cases with MMR mutations increased by at least one grade on recurrence (ΔGrade; 8/12), while only 28% of MMR-wild-type cases increased in grade on recurrence (14/50; P = .0119), suggesting that the development of an MMR mutation may in part drive progression to higher grade (Table 2). Here, IDH1 is linked to astrocytoma (excluding glioblastoma).