Medical illustration representing Huntington's disease, a genetic disorder involving the HD gene (Huntington Disease) of chromosome 4 with abnormal repetition of a sequence of 3 bases CAG (cytosine, adenine, guanine). This HD gene normally has a role protection of the brain but with this abnormal repetition it leads to the degeneration of neurons in the striatum (putamen and caudate nucleus) involved in the motor, cognitive and behavioral functions of the brain.