Inhibitory receptor acting as a major negative regulator of T-cell responses. 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. Homodimer; disulfide-linked (PubMed:11279501, PubMed:11279502, Ref.23, PubMed:21156796, PubMed:28484017). Binds to CD80/B7-1 and CD86/B7.2 (PubMed:11279501, PubMed:11279502, PubMed:28484017). Interacts with ICOSLG (PubMed:28484017). Exists primarily an intracellular antigen whose surface expression is tightly regulated by restricted trafficking to the cell surface and rapid internalization. Widely expressed with highest levels in lymphoid tissues. Detected in activated T-cells where expression levels are 30- to 50-fold less than CD28, the stimulatory coreceptor, on the cell surface following activation. N-glycosylation is important for dimerization. Phosphorylation at Tyr-201 prevents binding to the AP-2 adapter complex, blocks endocytosis, and leads to retention of CTLA4 on the cell surface. Genetic variations in CTLA4 are associated with susceptibility to several autoimmune disorders (PubMed:18595775, PubMed:12724780, PubMed:10189842, PubMed:10924276, PubMed:15138458, PubMed:15657618, PubMed:15688186, PubMed:25329329, PubMed:25213377). They influence responsiveness to hepatitis B virus (HBV) infection [MIM:610424] (PubMed:15452244). Disease susceptibility is associated with variants affecting the gene represented in this entry. Genetic variations in CTLA4 may influence susceptibility to Graves disease, an autoimmune disorder associated with overactivity of the thyroid gland and hyperthyroidism. Disease susceptibility is associated with variants affecting the gene represented in this entry. Disease susceptibility is associated with variants affecting the gene represented in this entry. The disease is caused by variants affecting the gene represented in this entry. Engineered fusion proteins consisting of the extracellular domain of CTLA4 and the IgG Fc region (Ctla4-Ig), inhibit T-cell-dependent antibody responses, and are used as immunosuppressive agents. They are soluble, have an enhanced affinity for B7 ligands and act as a competitive inhibitor of CD28. The therapeutic antibody Ipilimumab competes for the binding site of the endogenous ligands CD80/B7-1, CD86/B7-2 and ICOSLG. CLTA-4 entry