Human PD-1 (programmed death-1) is a 55 kDa member of the immunoglobulin superfamily that is induced in cells undergoing apoptosis. The PD-1 protein contains an immunoreceptor tyrosine-based inhibitory motif and is expressed predominantly on activated T and B lymphocytes. PD-1 plays a key role in peripheral tolerance and autoimmune diseases and is thought to be involved in the maintenance of peripheral self-tolerance by serving as a negative regulator of immune responses. Two novel members of the B7 family have been identified as PD-1 ligands, PD-L1 (B7-H1) and PD-L2 (B7-DC). Evidence reported to date suggests overlapping functions for these two PD-1 ligands and their constitutive expression on some normal tissues and up-regulation on activated antigen presenting cells.Synonyms: B7 homolog 1, B7H1, PD-L1, PDCD1 ligand 1, PDCD1L1, PDCD1LG1, Programmed cell death 1 ligand 1, Programmed death ligand 1