FMR1 and fragile X syndrome: FXS is caused by the lack of fragile X messenger ribonucleoprotein (FMRP), either due to an expansion of a CGG triplet (> 200 repetitions) in the 5’-untranslated region (UTR) of the Fmr1 gene that triggers DNA methylation and gene silencing (Sutcliffe et al. 1992; Verkerk et al. 1991) or due to missense mutations that impair the functionality of FMRP (De Boulle et al. 1993; Myrick et al. 2014).