Repeat expansions, such as those in HTT (CAG repeats in Huntington’s disease), ATXN1 and ATXN2 (CAG repeats in spinocerebellar ataxias), and FXN (GAA repeats in Friedreich’s ataxia), are well-established causes of neurodegenerative disorders19,20. The gene discussed is ATXN1; the disease is juvenile Huntington disease.