CD274 and cancer: PD-L1 is expressed in various types of normal cells (including placenta, vascular endothelium, pancreatic islet cells, muscle, hepatocytes, epithelium, and mesenchymal stem cells, as well as on B cells, T cells, DCs, macrophages, and mast cells), but it is overexpressed in many cancer cells, including melanoma, renal cell carcinoma, NSCLC, thymoma, and ovarian and colorectal cancer [129,131].