Abbreviations: CI = confidence interval, DM = diabetes mellitus, DN = diabetic nephropathy, ESRD = end-stage renal disease, HWE = Hardy-Weinberg equilibrium, MMP-9 = matrix metalloproteinase-9, NOS = Newcastle–Ottawa Scale, OR = odds ratio, PRISMA-P = Systematic Reviews and Meta-Analyses Protocols, SNP = single-nucleotide polymorphism. The gene discussed is MMP9; the disease is diabetes mellitus.