FMR1 and fragile X syndrome: For example, the fragile X syndrome (FXS) is the most frequent form of inherited XLID caused by an expansion of the CGG triplet repeat on X chromosome within the 5′-UTR of FMR1, which leads to the excessive methylation and silenced FMR1 transcription (Bell et al., 1991; Oberlé et al., 1991; Verkerk et al., 1991; Yu et al., 1991).