FMR1 and Intellectual disability: Fragile X syndrome (FXS), the leading monogenetic cause of intellectual disability (ID) and autism spectrum disorder (ASD), results from excessive trinucleotide cytosine–guanine–guanine (CGGn) repeats in the promotor region [1,2,3,4] of the fragile X mental retardation 1 (FMR1) gene.