Genome-wide association studies (GWAS) found many COPD risk candidate genes, such as CHRNA3/CHRNA5/IREB2, HHIP, FAM13A, RAB4B, EGLN2, MIA, CYP2A6, SOD3, MMP9, et al. [16]. This evidence concerns the gene MMP9 and chronic obstructive pulmonary disease.