HTT and Huntington disease: Neurodegeneration in HD is caused by the CAG repeat expansion of the huntingtin gene (HTT) encoding an abnormally long polyglutamine tract in the huntingtin protein within various brain areas such as the basal ganglia and cerebral cortex (Damiano et al., 2010; Ross and Tabrizi, 2011; Tabrizi et al., 2020).