The U.S. Food and Drug Administration (FDA) approved a new cancer therapy that involves genetically modifying a patient’s immune cells. The agency called the decision a “historical action” because the therapy, developed by Novartis, is the first gene therapy treatment approved in the United States. The new treatment involves removing immune cells called T cells from a patient’s blood and giving them a gene for a protein called a chimeric antigen receptor, or CAR, that directs the T cells to target leukemia cells. The modified cells are then transfused back. Although the treatment sometimes causes severe side effects, in a Novartis clinical trial the drug caused remission in 83% of 63 young patients with a certain type of leukemia, B cell acute lymphoblastic leukemia (ALL).