PDCD1 and cancer: Specifically, an understanding of immune response and activation in cancer, and in particular the role of immune blockade by CTLA-4 (cytotoxic T lymphocyte-associated protein 4) and PD-1/PD-L1 (programmed cell death protein 1/programmed cell death protein ligand 1) has revolutionized the treatment of several types of cancers, including but not limited to melanoma, non-small cell lung cancer, urothelial, head and neck, and renal cancers.