SR: Systematic Review, MA: Meta-analysis, CI: Confidence interval, p: p-value, I^2: the level of heterogeneity, r: correlation of coefficient, PHQ-9: Patient Health Questionnaire-9, RCT: Randomized control trial, HbA1C: Hemoglobin A1C, LSMD: least-squares mean difference, BDNF: brain-derived neurotrophic factor, TNF–α: Tissue necrosis factor-alpha, IL-1β: Interleukin-1beta, IL-6: Interleukin-6, IGF-1: Insulin Growth Factor-1, MDA: malondialdehyde, CRP: C-reactive protein, AOR: Adjusted Odds Ratio, DDD: Daily defined doses, BDI: Beck's Depression Inventory, FST: forced swim test. The gene discussed is BDNF; the disease is major depressive disorder.