IDH1 and Constitutional mismatch repair deficiency syndrome: Two children, with IDH1-mutant high-grade astrocytomas and germline heterozygous MSH2 variants, were diagnosed with primary MMR-deficient IDH1-mutant astrocytomas24 on the basis of Lynch syndrome, while the other two had bi-allelic MMR variants and were diagnosed with constitutional mismatch repair deficiency (CMMRD) syndrome.