CTLA-4 is a protein belonging to the immunoglobulin superfamily and the CD28 receptor family. It interacts with CD80 (B7-1) and CD86 (B7-2) to transmit inhibitory signals to T cells, in contrast to CD28, which sends stimulatory signals. This receptor is also found within regulatory T cells, where it plays a vital role in their functions. CTLA-4 is a key regulator of T cell activation, making it an appealing candidate for autoimmune disease therapy and cancer immunotherapy. Recent evidence further highlights its importance in the function and balance of regulatory T cells (Treg).