Cytotoxic T lymphocyte associated protein 4 (CTLA-4), also known as CD152, is a member of the immunoglobulin superfamily and is expressed on the surface of helper T cells. CTLA-4 is similar to the co stimulatory protein CD28 of T cells, both of which bind to CD80 and CD86. CTLA-4 transmits inhibitory signals to T cells, while CD28 transmits stimulating signals. CTLA-4 can function as an immune checkpoint, downregulate the immune system, and act as a “off” switch when binding to CD80 or CD86 on the surface of antigen presenting cells. Intracellular CTLA-4 is also present in regulatory T cells and plays an important role in their function. The mutation of this gene is related to insulin dependent diabetes, hyperthyroidism, Hashimoto’s thyroiditis, celiac disease, systemic lupus erythematosus, thyroid related ophthalmopathy, primary biliary cirrhosis and other autoimmune diseases. The polymorphism of the CTLA-4 gene is associated with autoimmune diseases such as autoimmune thyroid disease and multiple sclerosis, but this association is usually weak. In systemic lupus erythematosus, it has been found that splicing variants of soluble CTLA-4 are abnormally produced and present in the serum of patients with the disease.