Programmed Death-Ligand 1 (PD-L1; also known as B7-H1 or CD274), a transmembrane protein often overexpressed on the surface of various tumor cells and antigen-presenting cells within the immunosuppressive tumor microenvironment. Its primary physiological role is to deliver an inhibitory "off" signal to immune cells, particularly T cells, by engaging the PD-1 receptor and, to a lesser extent, the B7.1 (CD80) receptor. This interaction suppresses T cell activation, proliferation, cytokine production, and cytotoxic activity, leading to T cell exhaustion and allowing tumors to evade immune surveillance.