PDCD1 and cancer: Inhibitory programmed cell death 1 (PD-1), programmed cell death ligand-1 (PD-L1), or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) are the key immune checkpoints, and these pathways allow the malignant tumors to evade the host’s immunosurveillance (Marincola et al., 2003); therefore, their suppression activates the immune response against the cancer cells.