CD274 and neoplasm: The observed heterogeneity of CTLA-4, PD-L1 and PD-L2 expression reinforces the notion that multiple distinct phenotypes of tumor cells often co-occur within a patient and that the isolation of metastatic CTCs from peripheral blood can help characterize tumor cell populations for therapeutic targeting better than a tissue biopsy that could exclude some subpopulations, especially those that are dynamic and whose expression changes longitudinally with treatment [55].