Huntington’s Disease (HD) is an autosomal dominant neurodegenerative disorder characterized by involuntary movement, personality change and dementia. The HD gene was identified using gene linkage analysis.1 The HD gene contains a triplet (CAG) repeat expansion (>36 CAG repeats) in exon 1, something that is also a feature of polyglutamine diseases. A clear relation exists between the CAG repeat length of the HD gene and age of disease onset. HD’s pathological abnormalities include loss of medium spiny neurons in the striatum, especially the caudate nucleus and putamen, which can begin at an early stage of the disease. HD model mice, in the form of transgenic (Tg) or gene knock-in animals, have been produced by several groups. Several HD gene knock-in mice were also produced by replacing the mouse exon 1 with mutated forms of the human HD gene exon 1.2–6 In contrast to Tg mice, HD gene knock-in mice showed relatively mild abnormalities, including aggregation in nuclei of neurons, neuronal cell death and abnormal behavior.
