FXS is caused by a variable expansion of a trinucleotide (CGG) repeat in the 5′ UTR of the fragile X mental retardation-1 gene (FMR1), or less frequently, by point mutations in FMR1 (Collins et al., 2010; Handt et al., 2014), that leads to loss of FMR1 protein (Pieretti et al., 1991). The gene discussed is FMR1; the disease is fragile X syndrome.