PDCD1 and neoplasm: Immune checkpoint pathways, such as programmed cell death protein 1 (PD-1) and programmed cell death protein ligand 1 (PD-L1), can inhibit an effector T-cell antitumor immune response when it is upregulated in the tumor microenvironment, and therapies blocking this pathway have proven to be effective when improving an antitumor immune response [14,15].