We also controlled other confounding factors, include comorbidities such as CAD, CVD, CLD, COPD, CKD, GERD, and EE, and medications, such as metformin, sulfonylurea, glucosidase inhibitors, meglitinides, DPP-4 inhibitors, insulin, statins, ARBs, ACE inhibitors, aspirin, NSAIDs, and COX-2-specific inhibitors to minimize the limitations of nested case–control studies. This evidence concerns the gene DPP4 and chronic obstructive pulmonary disease.