FMR1 and fragile X syndrome: Fragile X syndrome (FXS) is the most common mental disorder caused by a CGG trinucleotide amplification on Xq27.3 in the 5′ untranslated region of fmr1 gene cloned and named in 1991, which suppresses production of fragile X mental retardation protein (FMRP) [1, 2].