glutamine repeats with histidine interruptions whose father had exhibited symptoms, but herself had not shown symptoms at the age of 66. Individuals with a low number of repeats, around 39 to 55, typically live past reproductive age and can pass on the disease to their children, while high repeats may express juvenile onset and fatality. Epidemiology The National Institute of Health reports that SCA1 has a prevalence of approximately 1 or 2 per 100,000 however a review of literature has shown that these estimates vary significantly from study to study and can be less than 1 per 100,000 or as high as 6 per 100,000. Among all types of SCA, SCA1 is among the most common and the portion accounted for by SCA1 varies between geographic regions, with percentages as high as 40% of all SCA diagnoses in populations in Russia and South Africa being SCA1. In the United States SCA1 accounts for 6% of SCA diagnoses. Overall, SCA1 accounts for 6-27% of all cases dominant ataxias. Because of its late onset, often appearing after reproductive age, SCA1 exerts low selection intensity, ranking about 0.19 on the Crow's index, but intensity may vary with time within a population or family, as anticipation increases the number of CAG repeats. One implication of this is that SCA1 is unlikely to disappear from a population by natural selection alone. Prevalence of each type of SCA varies with geographic region and ethnicity, possibly because of founder effects and historic migration patterns. High prevalence regions include central Poland, where 68% of autosomal dominant cerebellar disorders are SCA1; communities in Tamil Nadu, where up to 7.2% of the population has SCA1 in some small villages; the Tohoku region on the northern portion of Honshu Island, with 24.8% of cases being SCA1; and among Yakut populations in eastern Siberia, with a prevalence 46 per 100,000 in rural populations. History Ataxia as a symptom was first described by French neurologist Duchenne de Boulogne in a subject with tabes dorsalis. By the late 19th and early 20th centuries, extensive research into the characterization, cause, and diagnostics of hereditary cerebellar ataxias was underway with the work of several prominent neurologists, including Jean-Martin Charcot, Pierre Marie, Nikolaus Friedreich, Adolph Strümpell, and others. Marie described a number of cases of hereditary, adult onset disease he thought to be clinically distinct from Friedreich's ataxia, spastic paraplegia, and other known types of ataxia, calling