Fragile X (FXS) is the most common monogenic cause of ASD,38 and is a widely used and well-characterized model of ASD.37, 39 It results from silencing of the fragile X mental retardation (FMR1) gene and absence of its corresponding protein, the FXS mental retardation protein (FMRP). Here, FMR1 is linked to fragile X syndrome.