PDCD1 and cancer: The immune checkpoint receptors that have served as the primary targets of clinical cancer immunotherapy include the following: cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), programmed cell death protein 1 (PD-1), programmed cell death 1 ligand 1 (PD-L1), lymphocyte activation gene 3 (LAG-3), B and T lymphocyte attenuator (BTLA), and T cell immunoglobulin and mucin protein 3 (TIM-3) [13, 16].