HTT and Huntington disease: In order to decipher the normal biological function of Htt, critical to the understanding of HD pathology, several groups have observed the localization of Htt in vesicles, ER and nuclei in various cell models [2-4] and the N-terminal 18 amino acids of Htt were reported to constitute the membrane targeting domain that mediated the association of Htt with ER and late endosomes [2].