PDCD1 and cancer: Programmed death-ligand 1 (PD-L1) is highly expressed in diverse cancers, and is regarded as a major contributor to the immune incompetency of immune cells in cancer microenvironments through the ligation between PD-L1 and programmed cell death protein 1 (PD-1) on T cells that generate strong suppressive signals in the activated T cell to make it anergic [1,2,3,4,5].