FMR1 and fragile X syndrome: The Fmr1 knock-out mouse (Fmr1-KO) is an example of this approach: it is a confirmed model of Fragile X syndrome (FXS), a genetic disorder due to a mutation in the FMR1 gene leading to a lack of FMRP, a protein playing a pivotal role in synaptic functioning [3], [4].