Most cultured HD cell lines display limited CAG repeat instability, so we developed a model system for this purpose by inserting an expanded CAG repeat construct into the adeno-associated virus (AAV) integration site and safe harbor locus in intron 1 of PPP1R12C on chromosome 19 (AAVS1, Fig. 3a) of hTERT-RPE1 (RPE1) cells. This evidence concerns the gene PPP1R12C and Huntington disease.