For instance, as knocking down GPNMB gene down-regulates the expression of MMP9 [21], and as GPNMB directly stimulates expression of MMP9 in fibroblasts [22], one may infer that GPNMB would contribute to the tissue remodeling in COPD airways through promoting secretion of MMP9, which is, indeed, dramatically elevated in COPD [23]. This evidence concerns the gene GPNMB and chronic obstructive pulmonary disease.