Fragile X syndrome (FXS) is the most common genetic cause of autism spectrum disorder (ASD) and intellectual disability, occurring in approximately 1 in 4,000 males and 1 in 6,000 females. Fragile X syndrome is one of three disorders caused by a mutation on the X chromosome involving the FMR1 gene. Within that gene, a repeat segment of DNA called the CGG trinucleotide interferes with production of the gene product fragile X mental retardation protein (FMRP). This interference leads to the clinical manifestations of FXS as well as the other fragile X disorders, such as fragile X–associated tremor/ataxia syndrome (FXTAS) and fragile X–associated premature ovarian insufficiency (FXPOI). These disorders are together sometimes referred to as fragile X spectrum disorder (FXSD). Men affected by FXS almost always ...