PD-L1 expression refers to the presence of the programmed death ligand 1 protein on the surface of cells. This protein plays a crucial role in regulating the immune response by interacting with the programmed death receptor 1 (PD-1) on T cells, leading to T cell inactivation and suppression of the immune response. Research in this area focuses on understanding the role of PD-L1 expression in various diseases, particularly cancer. High levels of PD-L1 expression on cancer cells can help them evade the immune system and grow unchecked. Therapies targeting the PD-L1/PD-1 pathway, such as immune checkpoint inhibitors, have shown promising results in treating certain cancers by restoring the immune response against tumor cells. Studying PD-L1 expression can also help predict which patients are more likely to respond to immune checkpoint inhibitor therapy and guide treatment decisions. Researchers are exploring ways to detect PD-L1 expression levels in tumors, develop new therapies that target the PD-L1/PD-1 pathway, and improve patient outcomes through personalized medicine approaches.