FMR1 and fragile X syndrome: Fragile X syndrome (FXS) is typically the result of a hypermethlyated cytosine-guanine-guanine (CGG) trinucleotide repeat expansion in the 5’ UTR of the Fragile X mental retardation 1 gene (FMR1), leading to its silencing and subsequent loss of its protein product, fragile X mental retardation protein (FMRP).