FXS is caused by an expansion of CGG triplets (>200) in the 5′ untranslated region of the fragile X mental retardation gene 1 (FMR1), located on chromosome X (Krawczun et al., 1985; Fu et al., 1991), which results in the failure to produce the FMRP, required for normal neural development (Pieretti et al., 1991; Verheij et al., 1993; Santoro et al., 2012). Here, FMR1 is linked to fragile X syndrome.