CTLA4 is a member of the immunoglobulin superfamilyand encodes a protein which transmits an inhibitory signal to Tcells. The protein contains a V domain, a transmembrane domain, anda cytoplasmic tail. Alternate transcriptional splice variants,encoding different isoforms, have been characterized. Themembrane-bound isoform functions as a homodimer interconnected by adisulfide bond, while the soluble isoform functions as a monomer.Mutations in this gene have been associated with insulin-dependentdiabetes mellitus, Graves disease, Hashimoto thyroiditis, celiacdisease, systemic lupus erythematosus, thyroid-associatedorbitopathy, and other autoimmune diseases.