Spinocerebellar ataxia 2 (SCA2) is an autosomal dominant ataxia caused by abnormal expansion of unstable CAG repeat in a novel gene on chromosome 12q24.1. Size of the CAG repeat correlates inversely with age at onset. The clinical feature of SCA2 is affected with age at onset and duration of the disorder. In addition, not only rate of progression but also frequencies of slow saccade, hyporefiexia, dementia, tremor, or variety of extrapyramidal manifestations are known to correlate with the CAG repeat size. These correlation indicate that common molecular mechanisms underlie in the pathology of CAG triplet repeat disorders, including SCA2.