To test this hypothesis, we examined DNA methylation levels at nearly half a million sites throughout the genome in the peripheral blood and fibroblast iPS cells of FXS patients using a highly sensitive genome-wide assay that quantitates methylation level at single CpG dinucleotide resolution; our results show that the hypermethylation of the FMR1 locus in FXS is indeed locus-specific. Here, FMR1 is linked to fragile X syndrome.