MMP9 and chronic lung disease: Respiratory syncytial virus (RSV) infection has a substantial global disease burden.1,2,3,4 Matrix metalloproteinases (MMPs) are proteases implicated in the pathogenesis of both acute and chronic lung disease.5,6,7,8,9,10,11 In 2015, we reported12 that early elevation of endotracheal MMP-9 levels was positively associated with RSV disease severity and that MMP-9 inhibition decreased RSV replication in a murine model, suggesting that MMP-9 might be a potential marker of disease severity and a therapeutic target in RSV disease.