The responsible gene for FXS is the fragile X mental retardation 1 (FMR1) gene on chromosome Xq27.3, and an aberrantly expanded CGG repeat (> 200 repeats) in the 5’ untranslated region (UTR) of FMR1 causes the deficiency of the FMR1 protein (also called as FMRP), resulting in the onset of FXS7. The gene discussed is FMR1; the disease is fragile X syndrome.