IDH1 and Huntington disease: Astrocytoma, IDH-mutant with homozygous deletion of CDKN2A and/or CDKN2B (CDKN2A/B-HD) carries a poor prognosis [2–4], and CDKN2A/B-HD tumors are graded as “astrocytoma, IDH-mutant, CNS grade 4,” even in the absence of histological features of necrosis or microvascular proliferation [1].