PDCD1 and Merkel cell skin cancer: To date, various ICI agents, which target cytotoxic T-lymphocyte-associated protein 4, programmed cell death protein 1 (PD-1), and programmed death-ligand 1 (PD-L1), have been approved by the U.S. Food and Drug Administration to treat a variety of solid tumours, such as urothelial carcinoma, non-small cell lung cancer, hepatocellular carcinoma, and Merkel cell carcinoma.