The concentration of plasma MMP-9 active form as measured by gel zymography was higher in the FXS group as compared to controls (0.48 mg/l ± 0.28 vs. 0.27 mg/l ± 0.14; p < 0.05) (Fig. 3), while plasma total MMP-9 level assessed by ELISA showed no statically significant difference (Supplemental Table 1). Here, MMP9 is linked to fragile X syndrome.