FMR1 and fragile X syndrome: Furthermore, we were also unable to test the effect of MSH2 on de novo methylation in FXS cells after TET1 mediated demethylation of the FMR1 promoter because, in contrast to a previous report [51], reactivation of FXS alleles resulted in a high frequency of contractions even in cells lacking MSH2 (Fig. 7E).