Huntington's disease is an inherited disorder caused by expansion of a CAG 
trinucleotide repeat in the IT15 gene, which leads to expansion of a 
polyglutamine tract within the protein called huntingtin. Despite the 
characterization of the IT15 gene and the mutation involved in the disease, the 
normal function of huntingtin and the effects of the mutation on its function 
and on its neuronal location remain unknown. To study whether mutated huntingtin 
has the same neuronal distribution and intracellular location as normal 
huntingtin, we analyzed immunohistochemically both forms of this protein in the 
brain of 5 controls and 5 patients with Huntington's disease. We show that the 
distribution of mutated huntingtin is, like that of the normal form, 
heterogeneous throughout the brain, but is not limited to vulnerable neurons in 
Huntington's disease, supporting the hypothesis that the presence of the mutated 
huntingtin in a neuron is not in itself sufficient to lead to neuronal death. 
Moreover, whereas normal huntingtin is detected in some neuronal perikarya, 
nerve fibers, and nerve endings, the mutated form is observed in some neuronal 
perikarya and proximal nerve processes but is not detectable in nerve endings. 
Our results suggest that the expression or processing of the mutated huntingtin 
in perikarya and nerve endings differs quantitatively or qualitatively from the 
expression of the normal form in the same neuronal compartments.