In November 2023, US drug regulators initiated an investigation into the safety of CAR T-cell therapy, a groundbreaking cancer treatment that involves engineering a patient’s immune cells to target cancer. The FDA announced it was probing whether this therapy was linked to the development of new cancers, specifically immune-cell cancers called lymphomas, in individuals who had undergone the treatment. Bruce Levine, a pioneer in CAR T-cell therapy, was surprised by the news and began raising questions about the affected patients, their medical history, and prior treatments. As of March 25, the FDA had received 11 additional reports of secondary cancer, although none have been definitively linked to CAR T-cell therapy. However, some cases strongly suggest a potential risk associated with the treatment. Causality remains difficult to prove, as illustrated by a case study of a patient treated for multiple myeloma who later developed T-cell lymphoma, with the CAR gene found integrated into a regulatory region of another gene. This investigation underscores the need for thorough evaluation of the safety and long-term effects of innovative cancer therapies.