TNF and chronic obstructive pulmonary disease: Considering all of the factors we mentioned in “Data Extraction” comprehensively, 15 variants were deemed to have a significant association with COPD risk: TNF-α rs1800629 and rs1800630, GSTT1 null/present, MMP9 rs3918242, EPHX1 rs1051740 and rs2234922, IL-13 rs1800925, IL-6 rs1800796, ADAM33 rs2280091, rs511898, and rs612709, SP-D rs721917, VDBP 1F and 1S, and HHIP rs13118928.