Stimulating the body’s immune system to attack tumors is a promising way to treat cancer. Scientists are working on two complementary strategies to achieve that: taking off the brakes that tumors put on the immune system; and “stepping on the gas,” or delivering molecules that jumpstart immune cells. However, when jump-starting the immune system, researchers have to be careful not to overstimulate it, which can cause severe and potentially fatal side effects. A team of MIT researchers has now developed a new way to deliver a stimulatory molecule called interleukin 12 (IL-12) directly to tumors, avoiding the toxic effects that can occur when immunostimulatory drugs are given throughout the body. In a study of mice, this new treatment eliminated many tumors when delivered along with an FDA-approved drug that takes the brakes off the immune system.