Mouse Cytotoxic Tlymphocyte 4(CTLA-4,CD152), is a type I transmembrane T cell inhibitory molecule. mouse CTLA4 cDNA encodes 223 amino acids (aa) including a 35 aa signal sequence, a 126 aa extracellular domain (ECD) with one Ig-like V-type domain, a 21 aa transmembrane (TM) sequence, and a 41 aa cytoplasmic sequence. Within the ECD, Mouse CTLA-4 shares 68% aa sequence identity with human. CTLA4 is similar to the T cell costimulatory protein CD28 since both of the molecules bind to CD80 and CD86 on antigen-presenting cells. CTLA4 transmits an inhibitory signal to T cells, whereas CD28 transmits a stimulatory signal. Intracellular CTLA4 is also found inregulatory T cells and may play an important role in their functions. T cell activation through the T cell receptor and CD28 leads to increased expression of CTLA4. Genetic variations of CTLA4 have been associated with susceptibility to systemic lupus erythematosus(SLE), Gravesdisease(GRD), Celiac disease type3(CELIAC3) and Hepatitis B virus infection(HBVinfection).