MMP9 and chronic obstructive pulmonary disease: Sputum cell MMP-9 expression levels (Affymetrix number 203936_s_at, on U133 + 2 chips) were measured in RNA isolated from airway inflammatory infiltrate cells harvested from induced sputum in healthy never smokers (n = 15), healthy smokers (n = 13), smokers with moderate to severe COPD (n = 15) and ex-smokers with moderate to severe COPD (n = 12).