Fragile X Syndrome (FXS), the most common inherited intellectual disability and the leading mono-genetic cause of autism spectrum disorder (ASD)1,2, is a neurodevelopmental condition due to a CGG trinucleotide expansion in the fragile X mental retardation 1 (Fmr1) gene locus Xq27.3. The gene discussed is FMR1; the disease is autism spectrum disorder.