These monogenic syndromes, including Fragile X syndrome (FMR1), Rett syndrome (MECP2), PTEN hamartoma tumor syndrome (PTEN), tuberous sclerosis complex (TSC1/TSC2), Phelan McDermid syndrome (SHANK3), and cortical dysplasia focal epilepsy syndrome (CNTNAP2), play a key role in furthering our understanding of ASD. Here, FMR1 is linked to Down syndrome.