ASD is clinically and genetically heterogeneous with several known monogenic disorders presenting with ASD symptoms, such as Fragile X syndrome (FMR1), Tuberous Sclerosis (TSC1, TSC2), and Rett syndrome (MECP2), contributing 1%–5% to the ASD etiology (Betancur and Coleman, 2013). The gene discussed is FMR1; the disease is Down syndrome.