Microsatellite instability (MSI) is present in hereditary conditions due to 
mismatch repair (MMR) gene mutations. Following MSI analysis, tumor samples are 
classified into MSS (stable), MSI-L (low instability), and MSI-H (high 
instability) based on the fraction of unstable loci. Another MSI-based 
classification takes into account the size difference between mutant alleles in 
tumor DNA compared to wild-type alleles; two types of MSI, A and B, are 
recognized using this approach, type A being characterized by smaller, more 
subtle allelic shifts compared to type B. Biallelic mutations of MMR genes are 
associated with pediatric cancers, including glial tumors, in Turcot syndrome 
type 1 (TS1). However, most TS1-associated gliomas so far analyzed did not 
display MSI. We investigated the frequency of MSI in a series of 34 pediatric 
gliomas of different grade using a panel of five mononucleotide quasimonomorphic 
markers. Subtle qualitative changes were observed for the majority of markers in 
two glioblastomas (5.9% of the total series and 33.3% of glioblastomas). In both 
cases, family histories were compatible with TS1, and mutations of the PMS2 and 
MLH1 genes were identified. In one family, the MSI patterns were compared 
between the glioblastoma and a colon cancer from an affected relative, showing a 
clear qualitative difference, with the former displaying type A and the latter 
type B instability, respectively. These results were confirmed using additional 
microsatellite markers, indicating that knowledge of the association between 
TS1-related glial tumors and subtle type A MSI is important for full 
ascertainment of TS1 patients and appropriate counselling.