Bouton density in HD striatal neurons was significantly smaller than that in WT neurons (0.68 ± 0.033 boutons/μm for HD neurons (N = 8 experiments) versus 0.83 ± 0.034 boutons/μm for WT neurons (N = 6), p = 0.0012 from Mann–Whitney U test)) (Figure 2C), suggesting that the mutant huntingtin protein decreases the number of functional presynaptic terminals in HD striatal neurons. The gene discussed is HTT; the disease is Huntington disease.