PDCD1 and lung cancer: Immune checkpoint inhibitors (ICIs), including programmed cell death 1 (PD-1) or its ligand programmed cell death ligand 1 (PD-L1) and cytotoxic T-lymphocyte antigen 4 (CTLA-4) inhibitors, have made major breakthroughs in improving the progression-free survival (PFS) and overall survival (OS) rates of lung cancer patients (1).