Other studies have investigated the possibility of using siRNA in treating Huntington’s disease, which involves a protein called huntingtin. Huntingtin is a protein expressed in the brain and other tissues whose exact function is not known but which is thought to play an important role in signaling, transporting materials, binding proteins and other structures, and protecting against programmed cell death. The gene encoding this protein has a characteristic sequence of the three nucleotides, CAG, at one end. In healthy people, this sequence is repeated up to 35 times in a row. In people with Huntington’s disease, however, the gene carries more than 35 copies of this nucleotide sequence, resulting in an abnormal protein. To investigate the therapeutic silencing potential of the siRNA approach, researchers performed a single injection of siRNA to the huntingtin mRNA into the brain of mice expressing excessive levels of huntingtin protein containing 100 CAG repeats. This treatment prevented accumulation of the abnormal huntingtin protein (DiFiglia et al. 2007).