Higher MMP9 titres have been found in the sputum and saliva of individuals with chronic obstructive pulmonary disease (COPD) and asthma compared to healthy controls [31]; MMP9 is expressed in atherosclerotic plaques where it is believed to play a role in plaque rupture [32]; and MMP9 is involved in several neurological disorders, such as stroke, multiple sclerosis and chronic neuropathic pain [33]. This evidence concerns the gene MMP9 and chronic obstructive pulmonary disease.