A protein found on T cells (a type of immune cell) that helps keep the body’s immune responses in check. When CTLA-4 is bound to another protein called B7, it helps keep T cells from killing other cells, including cancer cells. Some anticancer drugs, called immune checkpoint inhibitors, are used to block CTLA-4. When this protein is blocked, the “brakes” on the immune system are released and the ability of T cells to kill cancer cells is increased.