HTT and Huntington disease: The toxic gain of functionof mtHTT is generally considered as the primary cause of disease [3,4], andaccordingly, it has been shown in mice that deleting the expanded allele ordecreasing the expression of the protein can halt the progression of HD [5,6].Furthermore, several studies suggest that reducing mtHTT, as well as wild-type HTT(wtHTT), is well tolerated in adult mice and larger animals [4,7–9], butthe loss of wtHTT is lethal to the mouse embryo [10].