FMR1 and Intellectual disability: Fragile X syndrome (FXS), the most common form of heritable intellectual disability, is caused by a disruption of the fragile X mental retardation 1 gene (FMR1; also called fragile X messenger ribonucleoprotein 1 gene) on the X chromosome and the resultant absence of FMR protein (FMRP) [1,2].