The gene mutation that causes Huntington’s disease appears in every cell in the body, yet kills only two types of brain cells. Why? Scientists used a unique approach to switch the gene off in individual brain regions and zero in on those that play a role in causing the disease in mice. Their findings shed light on where Huntington's starts in the brain. It also suggests new targets and routes for therapeutic drugs to slow the devastating disease, which strikes an estimated 35,000 Americans.