An autosomal dominant neurodegenerative disease manifesting in movement, cognitive and psychiatric disorders caused by the abnormal expansion of a polyglutamine (polyQ) stretch in the Huntingtin (HTT) protein. It mostly affects the caudate-putamen striatal neurons. Other areas are also affected and undergo a size reduction. HTT is thought to be involved in several cellular pathways. Its functions are overcome by the mutant protein mHTT.