PDCD1 and cancer: Since initiation of the first clinical trial of the anti-programmed cell death protein 1 (PD-1) monoclonal antibody (mAb) nivolumab in 2006, 6 mAbs targeting either PD-1 or its ligand, programmed cell death ligand 1 (PD-L1), have been approved by the US Food and Drug Administration (FDA) to treat 14 types of cancer [1].