MSH2 and fragile X syndrome: To test whether the differences observed in the levels of DNA methylation upon loss of MSH2 between DMPK alleles in DM1 ESCs and FMR1 alleles in FXS ESCs could be attributed to the differences in the overall CpG density, we decided to study the role of MSH2 in another repeat expansion disorder, FRDA, because the differentially methylated region upstream of expanded GAA repeat in FXN alleles is less CpG-rich (Figure S4) and the GAA repeat itself cannot be methylated.