FXS is caused by transcriptional silencing of the FMR1 gene coding for fragile X mental retardation protein (FMRP) (Verkerk et al., 1991), an mRNA-binding protein mostly functioning as a repressor (Garber et al., 2006) and in some cases as an enhancer (Bechara et al., 2009) of protein translation (Darnell and Klann, 2013). Here, FMR1 is linked to fragile X syndrome.