FMR1 and fragile X syndrome: FXS is caused by the absence of the fragile X mental retardation protein (FMRP) [4], [5], [6], which belongs to the RNA-binding, fragile X related protein (FXRP) family that includes also the fragile X related proteins 1 and 2 (FXR1P and FXR2P) [7], [8].