PD-L1,a 40kDa type 1 transmembrane protein, is a PD-1 ligand that can deliver inhibitory signals to PD-1+ T-cells to suppress immune responses. PD-L1 is widely expressed in cancer, where it contributes to immune evasion and facilitates tumor growth.

Immunotherapy for the treatment of cancer is rapidly evolving from therapies that globally and non-specifically simulate the immune system to more targeted activation of individual components of the immune system. More specifically, therapies that inhibit the interaction between PD-L1, present on the surface of tumor or antigen-presenting cells, and PD-1, present on the surface of activated lymphocytes, are generating much excitement and enthusiasm, even in malignancies that are not traditionally considered to be immunogenic.