PDCD1 and cancer: Activated T cells, which induce apoptosis of the target cancer cells, have an immune checkpoint protein called programmed cell death 1 (PD-1) on their surfaces, and the binding of one of the two ligands for this protein (PD-L1 and PD-L2) expressed on the surfaces of the cancer cells to PD-1 inhibits the cytotoxic activity of the T cells31.