Fragile X syndrome (FXS), also called Martin-Bell syndrome, is a non-Mendelian trinucleotide repeat disorder. FXS is the most prevalent inherited cause of mild to severe intellectual disability and the most common monogenic cause of autism spectrum disorder (ASD). It accounts for about one-half of cases of X-linked mental retardation and is the most common cause of mental impairment after trisomy 21 physical features include a long and narrow face with a prominent jaw, flexible fingers, large ears, and enlarged testicles in males. These features usually become more apparent in older children. About a third of these children have features of autism and delayed speech that is present from an early age. Hyperactivity and seizures are common. FXS is indirectly the result of the expansion of the cysteine-guanine-guanine (CGG) triplet repeat within the Fragile X mental retardation one gene (FMR1) located on the X chromosome. This CGG expansion silences FMR1 expression resulting in an abolished or greatly diminished expression of the fragile X mental retardation protein (FMRP). FMRP is necessary for the development of neuronal connections (synapses) and some ovarian functions. The lack of FMRP is the direct cause of FXS. Diagnosis is by molecular genetic testing, which determines the number of CGG repeats in the FMR1 gene. Normally, it is between five and 40 repeats. Individuals with 55 to 200 repeats have an FMR1 gene premutation (PM) and usually (but not always) have a normal intellect. Individuals with greater than 200 CGG repeats have a full mutation (FM) for FXS. Testing for PM carriers (by an FMR1 DNA test) allows for genetic counseling. Prenatal testing for FXS is an option often provided to women with a premutation or a full mutation.