PDCD1 and neoplasm: The immune checkpoint protein programmed cell death protein-1 (PD-1) expressed on CD4+ and CD8+ T cells and its ligand programmed death-ligand 1 (PD-L1) which is overexpressed by tumor cells are effective immune checkpoints that inhibit the cytotoxic function of CD8+ T lymphocytes (CTLs) transforming CTLs into exhausted T cells which are not well defined or possibly regulatory cells (Tregs) that suppress immune responses (1).