Nitric oxide synthases （NOSs） are a family of enzymes catalyzing the production of nitric oxide （NO） from L-arginine. NO is an important cellular signaling molecule. It helps modulate vascular tone, insulin secretion, airway tone, and peristalsis, and is involved in angiogenesis and neural development. Nitric oxide is mediated in mammals by the calcium-calmodulin controlled isoenzymes eNOS （endothelial NOS） and nNOS （neuronal NOS）. Nitric oxide synthase, inducible （iNOS） is an enzyme which is encoded by the NOS2 gene in humans and mice. INOS involved in immune response, binds calmodulin at physiologically relevant concentrations, and produces NO as an immune defense mechanism. In mice, the function of Nos2 in immunity against a number of viruses, bacteria, fungi, and parasites has been well characterized. Nos2 is important for protective immunity against CMV.