IDH1 and Constitutional mismatch repair deficiency syndrome: None of the adult patients with IDH-wildtype glioblastomas in this large cohort were found to have biallelic germline mutation in a mismatch repair gene indicative of constitutional mismatch repair deficiency (CMMRD) syndrome due to homozygous or compound heterozygous germline mutations in one of the mismatch repair genes, which is associated with the development of an IDH- and histone H3-wildtype diffuse pediatric-type high-grade glioma subtype arising during childhood in most instances [5, 43].