CD274 and neoplasm: PD-L1 immunohistochemistry (IHC) analysis of human lung and breast tumours has identified three distinct patterns of positive PD-L1 expression: malignancies with predominant epithelial tumour cell PD-L1 expression, those with infiltrating immune cell expression only, or tumours with PD-L1 on tumour and immune cells (Fig. 1a,b).