PDCD1 and neoplasm: Included in these interactions are lymphocyte-bound programmed cell death protein 1 (PD-1) and the tissue- or tumor-bound programmed death-ligand 1 (PD-L1), as well as the T-cell-bound cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) and cluster of differentiation 80 and 86 (CD80, CD86) proteins [61].