Aggregated mutant huntingtin (mHtt) is the most classic cellular pathological characteristic of HD; extra amplificated CAG repeats in exon 1 of huntingtin lead to polyglutamine (polyQ) extension at the N-terminal of Htt protein, and mutant Htt accumulates to cause neuronal loss (Figure 2; Macdonald et al., 1993; Mangiarini et al., 1996; Ha and Fung, 2012). Here, HTT is linked to Huntington disease.