A recent study16 has identified MSH3 as a modifier of cognitive function in Huntington’s disease rather than motor function, while we have previously shown in this cohort that variation in MSH3, specifically carrying a 3a allele, has a protective effect on a composite score of disease progression, which included cognitive and psychomotor function.24,27 It is currently hypothesized that MSH3 is introducing an expansion of the HTT CAG repeat in the process of repair. The gene discussed is MSH3; the disease is Huntington disease.