FXS occurs due to an expansion of > 200 CGG trinucleotide repeats in the promoter region of the fragile X mental retardation-1 (FMR1) gene at locus 27.3 on the long arm of the X chromosome affecting approximately 1 in 4000 males and 1 in 8000 females in the general population [14–16]. The gene discussed is FMR1; the disease is fragile X syndrome.