Clinical trials using patients' own immune cells to target tumors have yielded promising results. However, this approach usually works only if the patients also receive large doses of drugs designed to help immune cells multiply rapidly, and those drugs have life-threatening side effects. Now engineers have devised a way to deliver the necessary drugs by smuggling them on the backs of the cells sent in to fight the tumor. That way, the drugs reach only their intended targets, greatly reducing the risk to the patient.