Spinocerebellar ataxia type 1 (SCA1) is a rare autosomal dominant disorder, which, like other spinocerebellar ataxias, is characterized by neurological symptoms including dysarthria, hypermetric saccades, and ataxia of gait and stance. This cerebellar dysfunction is progressive and permanent. First onset of symptoms is normally between 30 and 40 years of age, though juvenile onset can occur. Death typically occurs within 10 to 30 years from onset. SCA1 is typically inherited from the parents in an autosomal dominant regime; the children of a person with the disease have a 50% chance of inheriting it themselves, and new mutations can occur in some cases. It is caused by an expanded number of trinucleotide repeats in the polyglutamine tract of the ATXN1 gene, which encodes the ataxin 1 protein. This expansion results in a larger than normal number of repeats of the nucleotide sequence cytosine, adenine, guanine, or CAG, in the gene which, in turn, results in a larger than normal number of consecutive glutamine amino acid residues in the protein. This mutant protein causes degradation in certain types of neurons, like Purkinje neurons, which are common in the cerebellum, spinal cord, and related parts of the brain. While the mechanism is not fully understood, it is suspected that changes in the interactions between ataxin 1 and other proteins result in a toxic gain of function. The mutation can be detected before or after the onset of symptoms by genetic testing.
In cases where SCAs are not present in family history or where family history is unavailable, testing for the 4 most common SCAs will return positive results for 50% of suspected SCA cases. : 11  Individuals who are at risk of inheriting SCA1 but are currently presymptomatic can also be screened with molecular genetic testing. Genetic testing
Genetic testing is the only definitive way to differentiate between spinocerebellar ataxia types because of the similarity between clinical characteristics of these diseases and the large  variance between cases. Genetic testing is available for many SCA types, including the relatively common types SCA1, 2, 3, 6, and 7; and the less common SCA8, 10, 12, 14, and 17. However, genetic testing is high in cost and has a low diagnostic yield, with positive diagnoses being found in only 24% of tests ordered by a subspecialist and 10% overall.Genetic testing can be administered at various stages of disease progression. When genetic testing is administered after the onset of symptoms, the test is said to be diagnostic; in adults prior to onset of symptoms it is presymptomatic, and the test can be performed for prenatal or preimplantation diagnoses. The European Molecular Quality Genetics Network (EMQN) recommends criteria for each type, which must be met before testing can begin. The EQMN recommends that labs receive a written clinical evaluation of symptoms by a neurologist and a disclosure of family history, or lack of history, before initiating diagnostic genetic testing.