IDH1 and central nervous system cancer: According to the 2021 WHO central nervous system (CNS) classification system, the ATRX (alpha-thalassemia/mental retardation, X-linked) retained and 1p/19q-codeleted group defines a WHO Grade 2 or Grade 3 1p/19q codeleted oligodendroglioma; ATRX lost and homozygous deletion of CDKN2A/B is sufficient to classify IDH-mutant glioma as WHO Grade 4, and those without CDKN2A/B deletion are WHO Grade 2 or 3 astrocytoma6,7.