FMR1 and fragile X syndrome: Fragile X syndrome (FXS), the most common inherited mental retardation, is caused when the expression of the fragile X mental retardation protein (FMRP)—an essential factor for the maintenance of synapses—is downregulated in the brain tissue due to aberrant repeat of CGG trinucleotide in the 5′ UTR of the gene [9,10].