CTLA-4 (Cytotoxic T-Lymphocyte-Associated Protein 4), also known as CD152, is a transmembrane protein expressed on the surface of activated T cells. It plays a role in the negative regulation of the immune response and is one of the key targets in immune checkpoint therapy. CTLA-4 shares the same ligands, B7 molecules (including CD80 and CD86), with CD28; however, unlike CD28, the binding of CTLA-4 to B7 molecules inhibits T cell activation. This inhibitory effect helps prevent excessive immune responses, but in the tumor microenvironment, tumor cells may exploit CTLA-4 to evade immune surveillance and attack.