FMR1 and fragile X syndrome: Fragile X syndrome (FXS), the most common single gene cause of inherited intellectual disabilities and autism, is characterized by a CGG-repeat expansion (>200 CGG repeats, full mutation) in the portion of the first exon of the fragile X mental retardation 1 gene (FMR1), which encodes the 5' UTR of the FMR1 mRNA.