PDCD1 and lung cancer: Since the food and drug administration (FDA) has approved the PD-1 inhibitor pembrolizumab alone as front-line single-agent therapy instead of chemotherapy in patients with NSCLC without an actionable driver mutation (ALK or EGFR) and expressing [PD-L1 Tumor Proportion Score (TPS) ≥ 1%], the study of PD-L1 expression levels in the different subtypes of lung cancer has been of major interest.