The expression level of aryl hydrocarbon receptor repressor (AHRR) significantly increased in GOLD I, II, and III, but there was no significant change compared with the control group in GOLD IV; PLA2G1B and phenylethanolamine N-Methyltransferase (PNMT) were significantly downregulated in all four stages, but no correlation with disease severity was observed; maltase-glucoamylase (MGAM) was significantly increased only in GOLD IV, the most severe case of COPD (Figures 3B-G). Here, PNMT is linked to chronic obstructive pulmonary disease.