PDCD1 and cancer: The predominant clinically utilized drugs target programmed death ligand 1 (PD-L1) found on cancer cells and antigen-presenting cells (APCs), programmed cell death protein (PD-1) present on the surface of lymphocytes, and cytotoxic T-lymphocyte associated protein-4 (CTLA-4) found on regulatory T cells (Tregs) or activated T cells.