Nitric oxide (NO) is an inorganic, gaseous, and reactive free radical that acts as a biologic mediator in processes such as neurotransmission, vasorelaxation, and cytotoxicity. In addition, it has antimicrobial and anti-tumoral activities. NO production is mediated by members of the nitric oxide synthase (NOS) family. The NOS enzyme catalyzes the oxidization of L-arginine into L-citrulline and NO. Several subtypes have been identified: two constitutive isoforms (type I brain/neuronal NOS, and type III endothelial) and one inducible isoform (iNOS, or type II).