CD274 and non-small cell lung carcinoma: The most intensely studied checkpoint pathways in NSCLC are the PD-1 pathways, which incorporate receptor PD-1 and its reciprocal ligands (programmed death-ligand 1 and 2, PD-L1 and PD-L2, respectively), and the cytotoxic T-lymphocyte antigen-4 (CTLA-4) pathways.