CTLA-4 (Cytotoxic T-Lymphocyte Antigen 4) is also known as CD152, is an Inhibitory receptor acting as a major negative regulator of T-cell responses. CTLA-4 is a member of the immunoglobulin superfamily, which is expressed on the surface of T cells and transmits an inhibitory signal to T cells. CTLA-4 and CD28 are homologous receptors expressed by both CD4+ and CD8+ T cells, which mediate opposing functions in T-cell activation. Both receptors share a pair of ligands expressed on the surface of antigen-presenting cells (APCs). The affinity of CTLA-4 for its natural B7 family ligands, CD80 and CD86, is considerably stronger than the affinity of their cognate stimulatory co-receptor CD28.

> 90% as determined by SDS-PAGE