Background：

This gene encodes a major histocompatibility complex， class II associated， type I transmembrane protein which belongs to the butyrophilin-like B7 family of immunoregulators. It is thought to be involved in immune surveillance， serving as a negative T-cell regulator by decreasing T-cell proliferation and cytokine release. The encoded protein contains an N-terminal signal peptide， two pairs of immunoglobulin-like domains， separated by a heptad peptide sequence， and a C-terminal transmembrane domain. Naturally occurring mutations in this gene are associated with sarcoidosis， rheumatoid arthritis， ulcerative colitis， inflammatory bowel disease， myositis， type 1 diabetes， systemic lupus erythematosus， acute coronary syndrome， and prostate cancer.