Similar to “De novo replication repair deficient glioblastoma, IDH-wildtype”, many (but not all) patients with primary mismatch repair deficient IDH-mutant astrocytomas have underlying Lynch syndrome with a heterozygous germline mutation in one of the mismatch repair genes and somatic inactivation of the remaining allele. The gene discussed is IDH2; the disease is astrocytoma (excluding glioblastoma).