The number of pack-years correlated with IS MMP-12+-macrophages in COPD smokers (R = 0.54, p < 0.05), COPD ex-smokers (R = 0.64, p < 0.05) and 'healthy' smokers (R = 0.78, p < 0.05). The gene discussed is MMP12; the disease is chronic obstructive pulmonary disease.