Although it is not known whether this particular pathway affects the cerebral circulation after chronic OSA, other pathological states negatively impact the cerebral circulation through activation of the renin–angiotensin system, upregulation of NADPH oxidase, and generation of superoxide.109 In addition to NADPH oxidase being a major source, superoxide also can be generated in the vascular system by mitochondria, cyclooxygenase, lipoxygenases, hydroxylases, and xanthine oxidase. Here, FMO5 is linked to obstructive sleep apnea syndrome.