In a promising form of immunotherapy known as CAR T-cell (chimeric antigen receptor) therapy, the patient’s T cells are engineered to better recognize and attack antigens on the surface of cancer cells. In treatments currently approved for use in battling lymphoma and leukemia, however, the therapy has a drawback: Amidst the cancer-killing frenzy, many engineered T cells become tainted with the remnants of cancer antigens, which causes them to turn on other T cells. This eventually depletes the body of cancer-fighting cells and opens the door for a recurrence of cancer. A new study, however, has identified a way to tame the self-destructive tendencies of these killer T cells.