PDCD1 and neoplasm: As we deepened our understanding of tumor antigens and immune–tumor interactions in the latter half of the 20th century, the emergence of agents targeting CTLA-4, programmed cell death protein 1 (PD-1), and programmed death-ligand 1 (PD-L1) pathways marked significant successes in treating a range of malignancies [5,6,7].