This differential localization phenotype occurs early in the cascade of events detected during the lifespan of Hdh knock-in mice, months before the appearance of huntingtin amino-terminal fragment, and fulfills the genetic criteria from genotype-phenotype studies in HD patients, including polyglutamine length progressiveness and striatal specificity, suggesting that it follows from the same property that triggers HD pathogenesis. The gene discussed is HTT; the disease is Huntington disease.