MMP9 and migraine disorder: In peripheral blood, neutrophils are the primary producers of MMP-9, which is a mediator of extracellular matrix degradation, angiogenesis, and chemokine-dependent leukocyte extravasation at the blood–brain barrier.1 Previous studies have noted increases in plasma MMP-9 during migraine attack, which we have replicated.4,17,23 Extending these findings, we have shown that MMP-9 expression by neutrophils may vary between migraine phases.