Nitric oxide (NO) is an inorganic, gaseous free radical that carries a variety of messages between cells. Vasorelaxation, neurotransmission and cytotoxicity can all be potentiated through cellular response to NO. NO production is mediated by members of the nitric oxide synthase (NOS) family. NOS catalyzes the oxidization of L-arginine to produce L-citrulline and NO. Two constitutive isoforms, brain or neuronal NOS (b or nNOS, type I) & endothelial cell NOS (eNOS, type III), and one inducible isoform (iNOS, type II), have been cloned. All NOS isoforms contain calmodulin, nicotinamide adenine dinucleotide phosphate (NADPH), flavin adenine dinucleotide (FAD), and flavin mononucleotide (FMN) binding domains. bNOS and eNOS share approximately 50% sequence homology and their enzymatic activity depends on binding to the calcium/calmodulin complex. Increases in intracellular calcium lead to the production of low levels of NO over a short time. bNOS is found in neurons, peripheral nerve cells, macula densa, and pancreatic islet cells. Alternate splicing specifically regulates bNOS in striated muscle. The translated protein, bNOS mu, is 34aa residues larger than bNOS from brain.