FMR1 and fragile X syndrome: Fragile X syndrome is the most prevalent hereditary cause of mental disability, whose principal hallmark is the dynamic expansion of more than 55–2000 (CGG)n trinucleotide repeats, which are located in the FMR1 gene’s first exon (d(CGG)n) and the 5′-UTR of the FMR1 mRNAs (r(CGG)n).