PDCD1 and neoplasm: In tumors, cells highly express programmed death ligand-1 (PD-L1), and over- expressed PD-L1 mediates evasion of tumor cells from the immune system through binding to programmed cell death 1 (PD-1) protein expressed by activated immune cells.[4] PD-1 blockades can fight against cancer cells through suppressing PD-1 binding to PD-L1 to reboot the immune system.