Three isozymes of nitric oxide synthase (NOS) have been identified. Their cDNA- and protein structures as well as their genomic DNA structures have been described. NOS I (ncNOS, originally discovered in neurons) and NOS III (ecNOS, originally discovered in endothelial cells) are low output, Ca²⁺-activated enzymes whose physiological function is signal transduction. NOS II (iNOS, originally discovered in cytokine-induced macrophages) is a high output enzyme which produces toxic amounts of NO that represent an important component of the antimicrobial, antiparasitic and antineoplasic activity of these cells. Depending on the species, NOS II activity is largely (human) or completely (mouse and rat) Ca²⁺-independent. In the human species, the NOS isoforms I, II and III are encoded by three different genes located on chromosomes 12, 17 and 7, respectively. The amino acid sequences of the three human isozymes (deduced from the cloned cDNAs) show less than 59% identity. Across species, amino acid sequences are more than 90% conserved for NOS I and III, and greater 80% identical for NOS II. All NOS produce NO by oxidizing a guanidino nitrogen of L-arginine utilizing molecular oxygen and NADPH as co-substrates. All isoforms contain FAD, FMN and heme iron as prosthetic groups and require the cofactor BH₄. NOS I and III are constitutively expressed in various cells. Nevertheless, expression of these isoforms is subject to regulation. Expression is enhanced by e.g. estrogens (for NOS I and III), shear stress, TGF-β1, and (in certain endothelial cells) high glucose (for NOS III). TNF-α reduces the expression of NOS III by a post-trancriptional mechanism destabilizing the mRNA. The regulation of the NOS I expression seems to be very complex as reflected by at least 8 different promoters transcribing 8 different exon 1 sequences which are expressed differently in different cell types. Expression of NOS II is mainly regulated at the transcriptional level and can be induced in many cell types with suitable agents such as LPS, cytokines, and other compounds. Whether some cells can express NOS II constitutively is still under debate. Pathways resulting in the induction of the NOS II promoter may vary in different cells. Activation of transcription factor NF-κB seems to be an essential step for NOS II induction in most cells. The induction of NOS II can be inhibited by a wide variety of immunomodulatory compounds acting at the transcriptional levels and/or post-transcriptionally.