FMR1 and fragile X syndrome: Although animal models of FXS [25,26], including Drosophila, mouse, rat, and zebrafish, are invaluable in understanding the functions of FMRP, there are limitations because the pathogenic mechanism of FXS in humans, in which trinucleotide repeat expansions lead to gene silencing, does not apply to other species [6,12,27,28,29].