Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), also known as CD152, ALPS5, CELIAC3, GRD4, GSE, or IDDM12, is a type of CD28 homolog molecule that plays an important role in regulating T cell activation. CTLA-4 is normally present in the vesicles of cells and is then transported to the immune synapse formed between T cells and antigen-presentation cells (APCs) and inhibits further activation of T cells by blocking T cell receptors and their signaling pathways.