It is worth to note that the relevance of MMP-9 levels for the physiopathology of FXS has been recently underlined by the rescue of several FXS related behaviors after a chronic treatment of Fmr1-KO with metformin that, interestingly, resulted in the reduction of MMP-9 levels in the Fmr1-KO mouse brain (Gantois et al., 2017). This evidence concerns the gene FMR1 and fragile X syndrome.