SCA3 is thus one of a group of disorders associated with the expansion of a glutamine-encoding CAG repeat, including the SCAs 1, 2, 7 and 17, DRPLA and Huntington disease (HD), that share a similar genetic mechanism and are assumed to also share a common toxic gain of function of the polyglutamine domain in the resultant protein (Bunting et al. 2022). The gene discussed is ATN1; the disease is Huntington disease.