FXN and fragile X syndrome: To test whether the role of MSH2 in the maintenance of DNA methylation seen at the DMPK locus in DM1 ESCs was conserved in two other REDs, FXS and FRDA, we made null mutations in MSH2 in FXS ESCs and FRDA iPSCs and examined the effect on DNA methylation upstream of the expanded repeats for FMR1 and FXN alleles respectively.