Subgroup analysis based on the BDNF measuring instruments (either China or USA) exhibited that there were significant differences in BDNF values between T2DM and HC (China: P = 0.05; USA: P < 0.001; Total: P < 0.001), with large heterogeneity (China: P < 0.001 and I2 = 100%; USA: P < 0.001 and I2 = 99%; Total: P < 0.001 and I2 = 99%) (Fig 3). The gene discussed is BDNF; the disease is type 2 diabetes mellitus.