NOS2 and major depressive disorder: However, the gene–gene analyses revealed that a risk of depression increased five-fold with genotypes such as T/T-T/T of SOD2 c.47T > C(p.Val16Ala)(rs4880) and NOS2 c.1823C > T(p.Ser608Leu)(rs2297518) polymorphisms (p = 0.013).