ATXN1 and spinocerebellar ataxia type 1: Here, we show substantial correlation between brain region-specific levels of expansion of the XDP CCCTCT repeat and the HTT CAG repeat, as previously observed in a similar comparison between expansion of the HTT CAG repeat and of the ATXN1 CAG repeat underlying spinocerebellar ataxia type 1 (SCA1) [28].