FMR1 and fragile X syndrome: Fragile X Syndrome (FXS) is a neurodevelopmental disorder that is caused by the loss of function mutation of the fragile X mental retardation 1 (Fmr1) gene on the X chromosome (reviewed in O’Donnell and Warren, 2002; Santoro et al., 2012; Online Mendelian Inheritance in Man ® [OMIM] 309550) resulting in lack of fragile X mental retardation protein (FMRP) expression (Fu et al., 1991; Pieretti et al., 1991).