Abbreviations: ADA = American Diabetes Association, COVID-19 = coronavirus disease 2019, DM = diabetes mellitus, DPP-4 = dipeptidyl peptidase-4, GRADE = Grading of Recommendations Assessment, MD = mean difference, MERS-CoV = Middle East respiratory syndrome coronavirus, PRISMA = Preferred Reporting Items for Systematic Reviews and Meta-analysis, RCT = randomized controlled trials, RR = relative risk, SARS-CoV-2 = Severe Acute Respiratory Syndrome Coronavirus 2, SMD = standard mean difference. The gene discussed is DPP4; the disease is diabetes mellitus.