FMR1 and fragile X syndrome: Large dynamic triplet expansions of CGG repeats of the fragile X mental retardation 1 gene on the X chromosome are heritable and cause fragile X syndrome, which occurs in 2% to 8% of ASD cases and in 1% to 2% of ID cases.25,26,27 An increasing spectrum of clinical presentations has been observed among carriers of triplet expansions in fragile X mental retardation 1of intermediate length.