PDCD1 and lung cancer: A blockade of the immune checkpoint markers, such as programmed cell death 1 (PD-1), programmed cell death 1 ligand 1 (PD-L1) and cytotoxic T-lymphocyte-associated antigen-4 (CTLA4), resulted in a significantly prolonged survival rate, indicating a systemic anti-tumor immune deficiency in lung cancers [10–12].