Studies of glioblastoma diagnosed in patients with constitutional mismatch repair deficiency (CMMRD), a paediatric cancer predisposition syndrome caused by germline pathogenic variants in both alleles of an MMR gene [238], have previously shown that increased MSI could not be detected despite loss of MMR protein expression in the tumour or detection of MSI-H in synchronous adenocarcinomas [239,240,241]. This evidence concerns the gene MRC1 and adenocarcinoma.