MMP-9 (Gelatinase B, 92-kD type IV collagenase, EC 3.4.24.35) is an MMP that is present in low quantities in the healthy adult lung, but much more abundant in several chronic neutrophil (PMN)-predominant inflammatory lung diseases such as chronic obstructive pulmonary disease (COPD) and cystic fibrosis (CF). Here, MMP9 is linked to chronic obstructive pulmonary disease.