ATXN3 and Ataxia: Machado–Joseph disease (MJD, also known as spinocerebellar ataxia type 3 or SCA3) is the most common dominant ataxia in the world and is caused by abnormal expansion of CAG repeats in a coding region of ATXN3, which produces an elongated polyglutamine (polyQ) tract in the Ataxin-3 protein103.