FMR1 and fragile X syndrome: Fragile X syndrome (FXS), resulting from CGG expansion in the fragile X mental retardation (FMR1) gene and functional loss of fragile X mental retardation protein (FMRP), is the most prevalent single gene cause of autism and heritable developmental disability, affecting 1 in 4000 males and 1 in 6000 females worldwide1.