FXS is caused by absence of the fragile X mental retardation protein (FMRP), most commonly due to a CGG trinucleotide repeat expansion in the 5′ untranslated region of the FMR1 gene, whose promoter becomes silenced after ∼10–12.5 weeks (Willemsen et al., 2002) of gestation by interaction of FMR1 mRNA with its associated trinucleotide repeat DNA (Colak et al., 2014). Here, FMR1 is linked to fragile X syndrome.