MMP9 and myeloid sarcoma: To the best of our knowledge, this is the first study that longitudinally analyzes serum levels of active MMP-2 and active MMP-9 with sensitive activity assay systems in a cohort of RRMS patients during the treatment with Natalizumab in an attempt to provide further insight into the real significance of gelatinases in MS pathology and their role in monitoring efficacy of treatment.