The function of CTLA4 (cytotoxic T-lymphocyte associated protein 4, Ensembl gene identifier ENSG00000163599) is as follows. Inhibitory receptor acting as a major negative regulator of T-cell responses (PubMed:11279501, PubMed:11279502, PubMed:16551244, PubMed:1714933, PubMed:18641304, PubMed:28484017). Acts as a decoy receptor: the affinity of CTLA4 for its natural B7 family ligands, CD80 and CD86, is considerably stronger than the affinity of their cognate stimulatory coreceptor CD28 (PubMed:11279501, PubMed:11279502, PubMed:16551244, PubMed:1714933, PubMed:28484017). {ECO:0000269|PubMed:11279501, ECO:0000269|PubMed:11279502, ECO:0000269|PubMed:1714933, ECO:0000269|PubMed:18641304, ECO:0000269|PubMed:28484017, ECO:0000303|PubMed:16551244}.