SMM provides an “all-inclusive” stimulus representative of the CF airway milieu because it contains (1) products from bacteria, factors from neutrophils, including neutrophil elastase, lysozyme, cathepsin G, and MMP9 [39]; (2) secretory products from airway epithelia and macrophages, including cytokines [39]; and (3) hundreds of peptides, mucins and purines [61]. Here, MMP9 is linked to cystic fibrosis.