FMR1 and fragile X syndrome: The molecular cause of FXS is the extensive repeat expansion of a CGG triplet (200 repeats in the full mutation) in the 5′ untranslated region (UTR) and consequential hypermethylation of the Fmr1 gene, finally leading to transcriptional silencing of the fragile X mental retardation protein (FMRP) (2–4).