HTT and Huntington disease: HD is primarily caused by the expansion of cytosine-adenine-guanine (CAG) trinucleotide repeats in the huntingtin (HTT) protein-coding gene HTT gene that leads to the formation of a mutant form of mHTT containing an abnormal number of glutamine amino acid repeats, known as polyglutamine (polyQ) in its amino terminus (N-terminus) [82].