HTT and Huntington disease: HD is caused by an expansion of the trinucleotide CAG within the huntingtin (HTT) gene (HDCRG, 1993), while there are several known genetic mutations in ALS such as superoxide dismutase 1 (SOD1), chromosome 9 open reading frame 72 (C9ORF72) and, trans-activation responsive RNA-binding protein (TARBP).