Programmed cell death ligand 1 (PD-L1) also known as cluster of differentiation (CD274) or B7 homolog1 (B7-H1) is a type 1 transmembrane protein involved in the regulation of cellular and humoral immune responses. The interaction of PD-L1 with its receptor PD-1 provides both stimulatory and inhibitory signals in regulating T cell activation and tolerance during pregnancy, tissue allografts, autoimmune disease and malignant transformation. The binding of PD-L1 to PD-1 induces apoptosis or exhaustion in activated T cells, and blockade of this interaction has been shown to enhance the antitumor activity of T cells. PD-L1 is frequently over expressed in placenta, and many human tumors such as melanoma, diffuse large B-cell lymphoma, and carcinomas of the lung, bladder, breast, kidney and colon.