CD274 and cancer: Over-expression of human PD-L1 by tumour cells has been shown to be a key mechanism by which cancers can evade detection by the immune system (Freeman et al. 2000) and has been seen on the surface of many different tumour types, including melanoma, non-small cell lung cancer and lymphoma (Konishi et al. 2004; Nakanishi et al. 2007).