Huntington's disease is a pathology caused by a mutation in the HTT gene located on the short arm of chromosome 4. This gene codes for a protein known as Huntingtin which is involved in various cellular processes as well as in embryonic development. In addition, this protein is expressed in the nervous system.
When this gene has less than 27 CAG triplet repeats it is correct. However, if a mutation were to occur, the number of CAG repeats would increase. When the CAG triplet of the HTT gene repeats more than 35 times is when Huntington's disease develops.