PDCD1 and neoplasm: For example, negative costimulatory signals mediated by cell surface proteins such as the programmed cell death protein 1 (PD1) with its programmed cell death ligand 1 or 2 (PDL1 or PDL2) or the cytotoxic T lymphocyte-associated protein 4 (CTLA4) inhibit the activation of T cells and thereby play a central role in anti-tumor immunity in many malignancies [23].