HTT and Huntington disease: HD is a monogenic autosomal dominant genetic disease caused by mutations in the huntingtin (HTT) gene, the abnormal copy of which will have a 50% of probability being passed to the offsprings.415 At the molecular level, tandem repeats of triplet CAG are abnormally expanded in the encoding region of HTT gene, resulting in an insertion of polyglutamine (polyQ) polypeptide in the N-terminal domain of the mutant HTT (mHTT) protein which drives the death of the brain cells.416–418 HD symptoms usually appear between the ages of 30 and 50, and are influenced by familial inheritance.