CHRNA5 and chronic obstructive pulmonary disease: To further explore epigenetic links between cigarette smoking and smoking-related lung diseases, we conducted MR [23] using four CpGs at these three gene loci (Table 2)—cg19696491 and cg22563815 for CHRNA5, cg03234777 for AMICA1, and cg26850624 for AHRR—with the methylation of the CpG as the exposure, cis mQTLs of these CpGs as the instrumental variables, and lung cancer or COPD as the outcomes [24].