Among serine proteases, specifically, neutrophil elastase (NE), dipeptidyl peptidase 4, cathepsin G, proteinase 3, cathepsin C, mast cell-derived tryptase and chymase are found to be associated with the severity of COPD [35,36,37,38,39,40]. This evidence concerns the gene DPP4 and chronic obstructive pulmonary disease.