We found a strikingly high incidence of mutations in the ATRX gene (α thalassemia/mental retardation syndrome X-linked), entirely restricted to IDH-mutant LGGs of astrocytic lineage—astrocytomas and oligoastrocytomas—and mutually exclusive with 1p/19q codeletion. The gene discussed is IDH2; the disease is astrocytoma (excluding glioblastoma).