ATXN7 and juvenile Huntington disease: SCA7 is caused by the abnormal expansion of CAG triplet repeats encoding a polyglutamine (polyQ) tract in ATAXIN-7 (ATXN7), and therefore belongs to the group of CAG/polyQ expansion disorders, which includes Huntington’s disease (HD) and other SCAs (SCA1-3, 6 and 17).