PDCD1 and cancer: Programmed cell death protein 1 (PD-1), another target for cancer immunotherapy, is also an inhibitory receptor on T-cells and expression of its ligand, PD-L1, by neoplastic cells, as well as by myeloid cells in the tumor microenvironment (2), is thought to be a major mechanism by which tumors evade killing by the immune system (6, 7).