NOS oxidizes a guanidine nitrogen of arginine releasing nitric oxide in the form of a free radical and citrulline. Nitric oxide thus generated acts as a messenger in diverse functions including vasodilation neurotransmission, anti-tumor and antipathogenic activities.. NOS is classified under three types: Type I NOS is 150-160kD and is found in neurons. It is constitutively expressed and is Ca2+/calmodulin dependent. It is called NOS-1, neuronal NOS (nNOS), brain NOS (bNOS), cerebral / constitutive / Ca2+regulated NOS (cNOS). bOS is present in skeletal muscles and its absence in Duchenne’s muscular dystrophy accounts for its symptoms of the disease. Type II NOS is 130kD protein known as macrophage NOS (mNOS) or inducible NOS (iNOS). It is Ca2+/calmodulin independent and is expressed in activated macrophages and stimulated glial cells. Type III NOS is a 135kD protein called endothelial NOS (eNOS).