Towards this goal we (i) used 3 independent techniques to validate the association of MMP-9 serum levels with Duchenne, (ii) we characterised the MMP-9 isoforms in DMD by gelatin zymography, (iii) we confirmed in 2 large longitudinal cohorts that MMP-9 levels increase over time and (iv) we studied MMP-9 levels in two clinical trials involving patients treated with drisapersen, a 2′-O-methyl-phosphorothioate antisense oligonucleotide able to skip exon 51 in the dystrophin pre-mRNA. This evidence concerns the gene DMD and Duchenne muscular dystrophy.