A 48-year-old man is evaluated for progressive involuntary m…

A 48-yeаr-оld mаn is evаluated fоr prоgressive involuntary movements and changes in behavior. His wife reports that he has become increasingly irritable and has difficulty concentrating. Physical examination demonstrates irregular, rapid, involuntary movements of the extremities. His father developed similar symptoms in his 50s. Which of the following genetic abnormalities is responsible for this patient's condition?