*3.5. Severity in COPD and Genetic Analysis*

All COPD patients were clustered according to the degree of disease severity following the GOLD guidelines. Patients in GOLD grades I and II were grouped as G1 (*n* = 149), and those in the disease stages GOLD III and IV were grouped as G2 (*n* = 37). No significant differences were found in any of the 3 SNPs after comparing the genotype frequencies between these two groups. These results are shown in Supplementary Table S1.
