Grade WHO II/III IDH-mutant gliomas, including oligodendrogliomas (IDH-O) with 1p/19q codeletion and astrocytomas (IDH-A) with intact 1p/19q, show differences in chromatin accessibility and gene expression that correlate with the extent of ATRX loss. The gene discussed is ATRX; the disease is astrocytoma (excluding glioblastoma).