Matrix metallopeptidase 9 (MMP9) and tissue metallopeptidase inhibitor 1 (TIMP1) regulate collagen deposition in the extracellular matrix (ECM) of the lungs, whereas high activity and imbalance of MMP9 and TIMP1 lead to collagen synthesis in the alveolar ECM and the occurrence and progression of pulmonary fibrosis. This evidence concerns the gene MMP9 and pulmonary fibrosis.