PDCD1 and neoplasm: As a molecular target, immune checkpoint blockade enhances anti-tumor immunity by blocking inherent immune down-regulating factors, such as cytotoxic t-lymphocyte antigen 4 (CTLA-4) [106]and programmed cell death 1 (PD-1) or their ligands, programmed cell death ligand 1 (PD-L1) (Table 1) [107].