Programmed Death-Ligand 1 (PD-L1), also known as CD274 or B7 Homolog 1 (B7-H1), is a transmembrane protein involved in suppressing the immune system and rendering tumor cells resistant to CD8 T cell-mediated lysis through binding of the Programmed Death-1 (PD-1) receptor. Overexpression of PD-L1 may allow cancer cells to evade the actions of the host immune system. In renal cell carcinoma, upregulation of PD-L1 has been linked to increased tumor aggressiveness and risk of death, and, in ovarian cancer, higher expression of this protein has lead to significantly poorer prognosis. PD-L1 has also been linked to systemic lupus erythematosus and cutaneous melanoma. When considered in adjunct with CD8 tumor-infiltrating lymphocyte density, expression levels of PD-L1 may be a useful predictor of multiple cancer types, including stage III non-small cell lung cancer, hormone receptor negative breast cancer, and sentinel lymph node melanoma.