LIG1 and Huntington disease: To test whether the LIG1 missense variant associated with delayed HD impacts somatic CAG expansion, we used CRISPR-Cas9-mediated homology-directed repair to introduce the same mutation as the human modifier (19AM3, rs145821638 C->A) into the mouse Lig1 gene, changing the LIG1 lysine residue at amino acid 843 to an asparagine (Fig. 4A).