MMP9 and chronic obstructive pulmonary disease: Although MMP-12 is known to be highly expressed in human COPD patients, model mice in the CS group exhibited small changes in MMP-12 expression relative to the Air group (fold change = 1.2), as previously reported21, indicating that other MMPs, such as MMP-9 and MMP-2, which are elevated during lung inflammation21, contributed to probe accumulation in the lungs of the disease mice.