FMR1 and fragile X syndrome: Fragile X syndrome (FXS), the most common form of inherited mental retardation, is caused by expansion of the rCGG trinucleotide repeat in the 5′ untranslated region (5′ UTR) of the fragile X mental retardation 1 (FMR1) gene, which leads to silencing of its transcript and the loss of the encoded fragile X mental retardation protein (FMRP) [1]–[6].