Huntington's disease. Huntington's disease (HD) is a genetic neurodegenerative disorder. Patients with HD experience progressive brain degeneration, especially in the caudate nucleus. Researchers examined the association between the molecule N-acetylaspartate (NAA, a brain-specific metabolite) in the caudate nucleus and caudate atrophy in a sample of 10 patients with very early-stage HD. Here are the patients' caudate NAA concentrations (in international units, IU) and caudate volumes (in milliliters, ml):17 Volume NAA Subject Volume