Spinocerebellar ataxia type 1 (SCA1), spinocerebellar ataxia type 2 (SCA2) and Huntington disease are autosomal-dominant genetically determined neurodegenerative diseases, caused by the expanded CAG repeats (polyglutamine, polyQ) at the ATXN1, ATXN2 and HTT genes, respectively. The gene discussed is HTT; the disease is juvenile Huntington disease.