FMR1 and fragile X syndrome: Compared to controls, children with FXS had significant higher methylation levels for all CpG sites examined (p < 3.3 × 10−7), and within the FXS group, lower FSIQ (WG corrected) was associated with higher levels of DNA methylation, with the strongest relationship found for CpG sites within FMR1 intron 1 (p < 5.6 × 10−5).