PMS1 and Huntington disease: We show the splice modulators also target PMS1, a known modifier of HD age-at-onset, suggesting that their suppression of CAG expansion is due to pseudoexon inclusion in PMS1. Inactivation of PMS1 in the cell model dramatically reduced HTT CAG expansion, supporting this gene or its products as potential targets for reducing somatic CAG repeat instability to treat the first stage of HD pathogenesis.