Both knockout and transgenic knockin mouse models for FXS have been created [15,16,18,20,22], but although FMR1 knockout mice have been extremely informative for studying the clinical pathology of FMRP deficiency, they fail to recapitulate the course of the disease in a developmentally regulated context because they are FMRP protein deficient from day 1 [108]. This evidence concerns the gene FMR1 and fragile X syndrome.