DS-MB-03795 recognizes mouse CD279, a 55kD cell surface protein which is a member of the CD28/CTLA-4 family, otherwise known as Programmed Death-1 (PD-1). CD279 is expressed predominantly on activated T- and B- lymphocytes and on a subset of thymocytes. Studies suggest that CD279, an immunoinhibitory receptor, plays a critical role in peripheral tolerance induction and prevention of autoimmune disease. Two members of the B7 family, B7-H1 (PD-L1) and B7-DC (PD-L2), have been identified as the ligands for CD279. DS-MB-03795 does not block the binding of both B7-H1-Ig and B7-DC-Ig fusion proteins to PD-1 transfected BHK cells.