FXS is one of the most frequent causes of intellectual disability (ID) and/or autism spectrum disorder (ASD) in males and is caused by CGG repeat expansions exceeding 200 repeats (full expansion) in the 5′ untranslated region (UTR) of the FMR1 (FMRP translational regulator 1) gene (MIM #309550) on chromosome X. Above this threshold, CpGs contained within the CGG repeats are usually methylated and associated with an absence of FMR1 expression [8], [9] (Figure 1A–B). The gene discussed is FMR1; the disease is autism spectrum disorder.