Definitive confirmation of Huntington disease is achieved by genetic testing showing an expanded CAG repeat in which gene?

• A. APOE gene
• B. HTT gene
• C. TP53 gene
• D. APP gene

Answer: (B)

Huntington disease is defined by a trinucleotide repeat expansion—specifically CAG repeats—in the HTT gene. The expanded repeats create a longer polyglutamine tract in the huntingtin protein, leading to neurodegeneration that underlies the movement, cognitive, and psychiatric features of the disease. Genetic confirmation comes from testing the HTT gene to measure the number of CAG repeats: normal alleles typically have about 10–35 repeats, while disease-causing expansions are generally above 36 repeats, with larger expansions often correlating with earlier onset due to anticipation. The other genes listed are associated with different conditions (APOE with Alzheimer's risk, TP53 with cancer suppression, APP with amyloid-related disorders) and do not cause Huntington disease.