Programmed death ligand 1 (PD-L1, B7-H1 or CD274) is a member of the growing B7 family of immune proteins that provide signals for both stimulating and inhibiting T cell activation. CD274 has been identified as one of two ligands for programmed death 1 (PD-1), a member of the CD28 family of immunoreceptors. CD274 is widely expressed in several organs such as heart, skeletal muscle, placenta and lung, and in lower amounts in thymus, spleen, kidney and liver. CD274 expression is upregulated in a small fraction of activated T and B cells and a much larger fraction of activated monocytes. CD274 expression is also induced in dendritic cells and keratinocytes after IFN-γ stimulation. CD274 expression is also upregulated in a variety of tumor cell lines. Interaction of CD274 with PD-1 results in inhibition of TCR mediated proliferation and cytokine production, suggesting an inhibitory role in regulating immune responses.. The CD274 - PD-1 pathway is involved in the negative regulation of some immune responses and may play an important role in the regulation of peripheral tolerance.