PDL1, also known as programmed death-ligand 1, is a protein that is expressed on the surface of certain cells, including cancer cells and immune cells. It plays a key role in regulating the immune response by binding to its receptor, PD1, on T cells and inhibiting their activity. This interaction can suppress the immune system's ability to recognize and attack cancer cells, allowing tumors to evade destruction. PDL1 has become an important target for cancer immunotherapy, as blocking the interaction between PDL1 and PD1 can help to unleash the immune system's ability to recognize and destroy cancer cells. Drugs that target PDL1 or PD1, known as checkpoint inhibitors, have shown promising results in treating various types of cancer, including melanoma, lung cancer, and bladder cancer. The expression of PDL1 in tumor cells is also used as a biomarker to predict the response to checkpoint inhibitor therapy. Patients with high levels of PDL1 expression in their tumors may be more likely to benefit from treatment with checkpoint inhibitors. As a result, PDL1 testing has become an important part of the decision-making process for cancer treatment.