CD274 and cancer: Apart from proteins essential for the identification of immune cell populations, we also included antibodies against targets that can be used to assess functional states, e.g., proliferative activity or expression levels of immune checkpoint-related molecules, such as CTLA-4, Tim-3, PD-1, and PD-L1, some of which have already been proposed as candidate biomarkers in cancer immunotherapy (Patel and Kurzrock, 2015).