Homozygous transgenic mice are viable and fertile. In these mice the human huntingtin transgenic protein is expressed widely in many tissues (identical to the endogenous huntingtin protein), with highest expression levels in the brain and testes. Electrophysiological abnormalities can be measured by 6 months. A behavioral phenotype is first detected at 7 months when evidence of mild hyperkinetic movement disorder is noticeable. This disorder is characterized by progressive spontaneous hyperactivity during the dark phase of open field-testing. By 12 months of age selective degeneration of medium spiny neurons in the lateral striatum is observed. This degeneration is associated with the translocation of N-terminal huntingtin fragments to the nucleus.