Definition: a generic term now increasingly used to describe autosomal dominant-inherited ataxias that have a progressive course. The terminology is regulated by the human genome organization, and each new gene locus, when found, is indicated by “SCA” followed by a number. Currently, at least 23 distinct types have been reported (SCA1–SCA23). All types closely resemble one another clinically and usually cannot be distinguished by phenotype alone. Pathologically, all types are caused by a variable combination of nerve cell loss in the cerebellum, basis pontis, olivary nuclei, substantia nigra anterior horns, and in the posterior thoracic nucleus. Formerly, diseases in this group were usually labeled “Marie ataxia” or “olivopontocerebellar atrophies.” SCA3 is now known as Machado-Joseph disease. Many of these disorders are due to expansions of the CAG sequence in various genes on various chromosomes, including chromosome 3p, 6p, 20p, 5q, 6q, 7q, 8q, 11q, 12q, 15q, 19q, and 22q.