A final area of interest is the opposing system of vasodilators and vasoconstrictors that control systemic vascular tone. Two major contributors to vasomotor tone in humans are the endothelium derived vasodilator nitric oxide (NO) and the powerful vasoconstrictor endothelin-1. Endothelial NO is produced by the enzyme endothelial nitric oxide synthase (eNOS). A single base-pair polymorphism (G894T) is associated with an increased risk of vasospastic angina pectoris and with increased vascular responsiveness to phenylephrine in patients on cardiopulmonary bypass. Meanwhile, a single base-pair polymorphism in the preproendothelin-1 (pp-ET1) gene (G5665T) has been shown to increase vasomotor reactivity in human mammary arteries. This polymorphism is also associated with hypertension in overweight patients with the T allele.