HTT and Huntington disease: In Huntington disease (HD), the cardinal pathologic features are the worsening, topographic, degeneration of the neostriatum (caudate nucleus, putamen, and nucleus accumbens), in which the medium spiny neuron is the most vulnerable cell type, and the appearance of huntingtin inclusions due to abnormal polyglutamine (polyQ) expansion of the huntingtin protein (HTT) [13, 63, 64].