CD279 is a 50-55 kD immunoglobulin superfamily member, also known as programmed death-1 (PD-1). PD-1 is expressed on a subset of CD4-CD8- thymocytes, and on activated T and B cells. PD-1 is thought to be involved in lymphocyte clonal selection and peripheral tolerance. The PD-1 ligands, PD-L1 (also known as B7-H1) and PD-L2 (B7-DC), are members of the B7 immunoglobulin superfamily. The RMP1-14 antibody has been reported to block the binding of PD-1 to its ligands (B7-H1 and B7-DC) and to inhibit T cell proliferation and cytokine production costimulated by macrophages (but not by dendritic cells and B cells).