PDCD1 and neoplasm: Clinical data indicate that antagonists of cytotoxic T lymphocyte antigen 4 (CTLA-4) and programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) can lead to durable anti-tumor immune responses [1–5], and have led to widespread interest in targeting other T cell surface antigens, including co-stimulatory members of the tumor necrosis factor receptor superfamily (TNFRSF) such as OX40 and 4-1BB (CD137) [6, 7].