A total of 68 studies (Figure 1) met our inclusion criteria (involving 15,988 T2DM cases and 25,452 controls), of which 36 studies reported the eNOS 4b/a (8,553 cases and 6,613 controls), 44 studies investigated the eNOS G894T (10,722 cases and 21,256 controls), and 13 studies reported the eNOS T786C (4,676 cases and 3,842 controls), as shown in Tables 2–4. The gene discussed is NOS3; the disease is type 2 diabetes mellitus.