HTT and Huntington disease: Huntington’s disease (HD), a neurodegenerative disease characterized by progressive motor and cognitive dysfunction, is caused by the mutant huntingtin (mHtt) protein, which is encoded by the huntingtin (HTT) gene that has an expanded CAG repeat (> 36) in chromosome 4 (Dayalu and Albin, 2015; Li et al., 2015; Manoharan et al., 2016; Southwell et al., 2017).