NOS2 and major depressive disorder: For example, MAOA and MAOB are the widely distributed mitochondrial enzyme with high expression levels in gastro-intestinal and hepatic as well as neuronal tissues, and are genetically associated with the pathogenesis of mental disorders (Lin et al., 2000); In addition, NOS1 and NOS3 can play a role in the pathogenesis and symptom of depression, NOS2 is generally up-regulated in various tissues under inflammatory conditions (Chakrabarti et al., 2012).