Nitric oxide is synthesized by three isoenzymes widely distributed in the organism. The three genes encoding these enzymes show structural homology confirming that they are members of a protein family. Isoforms I (neuronal) and III (endothelial) are constitutive but their expression is transcriptionnaly regulated by various factors. NOS I promoter has not been studied yet, but alternative splicing around exons 9 and 10 has been described. SP1 binding on NOS III promoter is critical for its constitutive expression. NOS II isoform is inducible in a large variety of cells and seems also to be present constitutively in some tissues such as kidney. Functional studies of NOS II promoter reveal two important regions for lipopolysaccharides (- 85 à - 75) and interferon gamma (- 900 à - 975) induction. NOS III polymorphic markers allowed genetic studies which indicate that NOS III gene is not associated with human essential hypertension.