FMR1 and fragile X syndrome: FXS is caused by the transcriptional silencing of the Fragile X Messenger Ribonucleoprotein 1 (FMR1) gene due to the hypermethylation of a CGG repeat expansion in its 5’-untranslated region (5’UTR; (Oberlé et al., 1991; Verkerk et al., 1991; Vincent et al., 1991; Yu et al., 1991).