ERCC6 and Huntington disease: However, knockouts of different NER factors in different CAG repeat disease models have variable effects: Csb (Ercc6) knockout in HD mice promotes germline repeat instability (Kovtun et al., 2011); XPG (mus201) knockout in a Drosophila model of SCA3 abolishes repeat instability (Jung and Bonini, 2007); Xpa knockout in a SCA1 mouse reduces somatic repeat instability in many areas of the brain (although not in the cerebellum) (Hubert et al., 2011).