PDCD1LG2 and metastatic prostate carcinoma: In this study, we have characterized four such immune checkpoint biomarkers on CTCs (PD-L1, PD-L2, B7-H3, and CTLA-4) from men with metastatic prostate cancer and described the clear heterogeneity of each IC ligand on CTCs across metastatic prostate cancer disease states and between and within men over time.