In addition, COPD susceptibility- or disease progression-related genes have been reported [20], and genome-wide association studies (GWASs) have revealed single nucleotide polymorphism (SNP) sites related to COPD occurrence and development, such as HHIP [21], IL13 [22], MMP9 [23], SFTPB [24], SOD3 [25], CHRNA3 [26], RNF150 [27], BICD1 [28], COL4A3 [29], AQP5 [30], AGPHD1 [31], IREB2 [32], etc. Many studies have indicated that some candidate genes are associated with COPD over the past few years. Here, MMP9 is linked to chronic obstructive pulmonary disease.