In Huntington chorea the causative mutation in the protein huntingtin is a:

Huntington disease (HD) Mutation in the HTT gene which codes for protein called huntingtin which is impoant to neurons. The HTT mutation involves a DNA segment known as a CAG trinucleotide repeat. Made up of a series of three DNA building blocks (cytosine, adenine, and guanine) that appear multiple times in a row leading to the disease.