HD is caused by a CAG expansion in the gene encoding for huntingtin (htt), located on chromosome 4; normally, the htt gene contains up to 35 CAG repeats, while in HD it has more than 36 CAG repeats that produce a mutant protein, with an abnormally long polyglutamine repeat (polyQ), responsible for the selective striatal degeneration of medium-sized spiny neurons and cerebral cortex [170]. The gene discussed is HTT; the disease is Huntington disease.