In Fmr1 knockout (KO) mouse model of FXS, metformin reverses the increased grooming and social behavior deficits, rescues long-term depression and impaired spine morphology and selectively normalizes extracellular regulated kinase (ERK) signaling and the expression of matrix metalloproteinase-9 (MMP-9) [85]. Here, FMR1 is linked to fragile X syndrome.