In this report, we discuss 2 institutional cases of WHO grade 4 IDH-mutant astrocytoma with mosaic IDH1 R132H expression by immunohistochemistry and very low IDH1 variant allele frequencies (VAF) compared to other identified pathogenic mutations, including TP53 and/or ATRX, corresponding to subclonal IDH1 mutation (ie, a low IDH1 mutation TCF). The gene discussed is ATRX; the disease is astrocytoma (excluding glioblastoma).