CD152, also known as Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4), is a 33kD member of the immunoglobulin superfamily. It is transiently expressed on activated T cells. Regulatory T cells express high level of CTLA-4. CD152 is similar to CD28 in aa sequence, structure, and genomic organization. Whereas CD28 delivers a costimulatory signal in T cell activation, CTLA-4 negatively regulates cell-mediated immune responses through interaction with CD80 and CD86 present on antigen presenting cells (APC). CD152 is thought to play a role in the induction and maintenance of immunological tolerance, as well as the development of protective immunity and thymocyte regulation.