PDCD1 and neoplasm: Cytotoxic T Lymphocyte antigen 4 (CTLA-4), Programmed Cell Death 1 (PD-1), and Programmed Cell Death Ligand 1 (PD-L1, also known as B7-H1 or CD274) pathways are the best known immune checkpoint pathways for immune responses within the tumor microenvironment.