CTLA4 (CD152) is a cell surface receptor that behaves as a negative regulator of the proliferation and the effector function of T cells. It contains a V domain, a transmembrane domain, and a cytoplasmic tail. Alternate transcriptional splice variants, encoding different isoforms, have been characterized. The membrane-bound isoform functions as a homodimer interconnected by a disulfide bond, while the soluble isoform functions as a monomer. Mutations in this gene have been associated with insulin-dependent diabetes mellitus, Graves disease, Hashimoto thyroiditis, celiac disease, systemic lupus erythematosus, thyroid-associated orbitopathy, and other autoimmune diseases.The novel finding that CTLA-4 molecule is expressed and functional on human tumor cells opens up the possibility of antitumor therapeutic intervention based on targeting this molecule.