ATXN2 and Huntington disease: In the HD-associated region, ATN1 and ATXN2 shared co-expression partners with HTT and both genes were also functionally related to HTT. For each of the genes it was shown before that variations in the normal CAG repeat length, together with the expanded CAG repeat in HTT, affect the AAO in HD (Hmida-Ben Brahim et al., 2014).