ATXN3 and juvenile Huntington disease: PolyQ diseases is a group of disorders caused by CAG repeat expansions within the, respectively, responsible genes, including Huntington disease (HD), dentatorubral-pallidoluysian atrophy (DRPLA), spinocerebellar ataxias (SCA1, SCA2, SCA3/Machado-Joseph disease, SCA6, SCA7, and SCA17) [1–3], and the recently discovered Huntington disease-like 2 (HDL2) [4, 5].