Studies showed controversial results regarding the relationship between the activity of MMP-9 and lung function measured using the Tiffeneau-Pinelli index (FEV1/FEV ratio) [21, 22], suggesting the urgent need to understand the mechanisms underlying MMP-9 expression, activation and its regulation of airway remodeling in COPD. The gene discussed is MMP9; the disease is chronic obstructive pulmonary disease.