FMR1 and fragile X syndrome: Fragile X syndrome (FXS), the most common inherited cause of intellectual disability (ID) and the single leading monogenic currently known cause of Autism Spectrum Disorder (ASD) [1], is caused by an expansion of a CGG trinucleotide repeat, greater than 200, in the 5’ untranslated region of the fragile X mental retardation1 (FMR1) gene.