Endothelial nitric oxide synthase (eNOS), also known as nitric oxide synthase 3 (NOS3), is a critical enzyme primarily expressed in vascular endothelial cells, as well as in other tissues such as the myocardium, airway epithelium, and renal tubular cells. It catalyzes the conversion of L-arginine to L-citrulline, producing nitric oxide (NO) as a byproduct, which is a key signaling molecule for regulating vascular tone, promoting vasodilation, and maintaining cardiovascular homeostasis. eNOS activity is regulated by various stimuli, including shear stress, calcium influx, and phosphorylation at specific residues such as serine 1177. Dysfunction of eNOS, such as uncoupling due to oxidative stress or deficiency of cofactors like tetrahydrobiopterin (BH4), can lead to reduced NO production and contribute to endothelial dysfunction, a hallmark of cardiovascular diseases.