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Huntington's disease, one of more than 50 inherited repeat expansion disorders, is a dominantly inherited neurodegenerative disease caused by a CAG expansion in HTT. Inherited CAG repeat length is the primary determinant of age of onset, with human genetic studies underscoring that the disease is driven by the CAG length-dependent propensity of the repeat to further expand in the brain. Routes to slowing somatic CAG expansion, therefore, hold promise for disease-modifying therapies. Several DNA repair genes, notably in the mismatch repair pathway, modify somatic expansion in Huntington's disease mouse models. To identify novel modifiers of somatic expansion, we used CRISPR-Cas9 editing in Huntington's disease knock-in mice to enable in vivo screening of expansion-modifier candidates at scale. This included testing of Huntington's disease onset modifier genes emerging from human genome-wide association studies as well as interactions between modifier genes, providing insight into pathways underlying CAG expansion and potential therapeutic targets.