The aim of this study was therefore to investigate the in vivo role of MMP-9 in mice with airway-specific overexpression of the β-subunit of the epithelial Na+ channel (βENaC-Tg) that exhibit CF-like airway surface dehydration and impaired mucociliary clearance, and develop spontaneous lung disease that shares key features of patients with CF including early onset airway mucus plugging, chronic neutrophilic inflammation and structural lung damage [26,27,28,29]. Here, MMP9 is linked to cystic fibrosis.