MTOR and fragile X syndrome: A key pathogenetic factor of inherited mental retardation is the fragile X syndrome (FXS), caused by the mutated fragile X mental retardation gene (FMR1) that encodes the fragile mental retardation protein (FMRP), an inhibitor for PI3K enhancer (PIKE, an upstream stimulator of PI3K within the mTOR pathway) [52].