Fragile X syndrome (FXS), typically caused by an expansion of the CGG triplet repeat (>200) in the fragile X messenger ribonucleoprotein 1 (FMR1) gene on Xq27.3, is the most common inherited form of intellectual disability, and the leading form of XLID [2]. The gene discussed is FMR1; the disease is cask-related x-linked intellectual disability.