Huntington’s disease gene carriers with one or more 3a alleles in MSH3 had a slower rate of total GM atrophy over 3 years compared with non-carriers [for average age and DBS slope (95% CI) = −0.181 (−0.202, −0.160) and −0.122 (−0.148, −0.097)] for non-carriers and carriers, respectively; contrast estimate (SE) for non-carriers versus carriers = −0.0586 (0.0168), t(196) = −3.486, P < 0.001). This evidence concerns the gene MSH3 and Huntington disease.