FMR1 and fragile X syndrome: FXS results from a trinucleotide repeat (CGG) expansion mutation of more than 200 repeats (full mutation) in the promoter of FMR1 (Fragile X mental retardation 1 gene), which leads to transcriptional silencing of FMR1, and loss or significant reduction of expression of the gene product, the Fragile X mental retardation protein (FMRP).