PDCD1 and neoplasm: In addition, recognition of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and PD-1 co-inhibitory receptors, found on the surface of cytotoxic T cells (CTLs), with their tumor cell surface partner proteins B7-1/B7-2 and PD-L1, respectively, causes a lack of suitable T cell priming and activation, putting brakes on unrestricted cytotoxic T effector function against malignant cells [108,109,110].