Moyamoya disease is a disease in which certain arteries in the brain are constricted. Blood flow is blocked by constriction and blood clots (thrombosis). A collateral circulation develops around the blocked vessels to compensate for the blockage, but the collateral vessels are small, weak, and prone to bleeding, aneurysm and thrombosis. On conventional angiography, these collateral vessels have the appearance of a "puff of smoke" (described as "もやもや (moyamoya)" in Japanese). When moyamoya is diagnosed by itself, with no underlying correlational conditions, it is diagnosed as moyamoya disease. This is also the case when the arterial constriction and collateral circulation are bilateral. Moyamoya syndrome is unilateral arterial constriction, or occurs when one of the several specified conditions is also present. This may also be considered as moyamoya being secondary to the primary condition. Mainly, occlusion of the distal internal carotid artery occurs. On angiography, a "puff of smoke" appearance is seen, and the treatment of choice is surgical bypass. Presentation Patients usually present with TIA, ischemic/hemorrhagic stroke, or seizure. The age distribution is bimodal being either young adolescence or mid-forties. Cause About 10% of cases of moyamoya disease are familial, and some cases result from specific genetic mutations. Susceptibility to moyamoya disease-2 (MYMY2; 607151) is caused by variation in the RNF213 gene (613768) on the long arm of chromosome 17 (17q25). Moyamoya disease-5 (MYMY5; 614042) is caused by mutation in the ACTA2 gene (102620) on the long arm of chromosome 10 (10q23.3); and moyamoya disease-6 with achalasia (MYMY6; 615750) is caused by mutation in the GUCY1A3 gene (139396) on the long arm of chromosome 4 (4q32). Loci for the disorder have been mapped to the short arm of chromosome 3 (MYMY1) and the long arm of chromosome 8 (8q23) (MYMY3; 608796). See also MYMY4 (300845), an X-linked recessive syndromic disorder characterized by moyamoya disease, short stature, hypergonadotropic hypogonadism, and facial dysmorphism, and linked to q25.3, on chromosome 17. In the United States moyamoya has an incidence rate of 0.086 per 100,000. In Japan the overall incidence is higher (0.35 per 100,000). In North America, women in the third or fourth decade of life are most often affected, but the condition may also occur during infancy or childhood. These women frequently experience transient ischaemic attacks (TIA), cerebral hemorrhage, or may not experience any symptoms at all. They have a higher risk of recurrent stroke and may be experiencing a distinct underlying