Fragile X syndrome is the most common form of inherited mental retardation. Incidence frequency is 1/1200 in males and 1/2500 in females. It has been localized at Xq27.3 region. Syndrome appears with the three nucleotide repeats in FMR1 gene.

Although 6-40 repeats accepted as normal , 29 repeats is the most frequent one within the society. Repeats in between 40-55 is called as ‘’gray zone’’ and repeats in between 55-200 is accepted as permutation.