PDCD1 and neoplasm: The interaction of PD-1 with its ligands, programmed death-ligand 1 (PD-L1) and programmed death-ligand 2 (PD-L2), can deliver an inhibitory signal and limit effector T cell responses to protect tissues from immune-mediated damage [2], whereas tumor cells take advantage of this signal as a mechanism for immune escape[3]. The high expression of PD-1 is considered to be a marker of functional T cells [4] or exhausted T cells [5], and T cell exhaustion can be reversed by blocking PD-1 signaling [6].