CD279 is a 55 kD member of the immunoglobulin superfamily and contains the immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic region. CD279 plays a key role in peripheral tolerance and autoimmune disease and is expressed in activated T cells, B cells, and myeloid cells. The ligands of CD279 include PD-L1 and PD-L2, and there is evidence that these ligands have overlapping functions and expression in normal tissues.