To this end, the main objective of this study was to describe the expression of PD-L1, PD-L2, B7-H3, and CTLA-4 on CTCs isolated from men with metastatic prostate cancer across different hormone sensitive and castration resistant disease states and over time during treatment and disease progression, for the purposes of future therapeutic targeting and prognostic biomarker studies of immune checkpoint inhibition in men with metastatic prostate cancer. The gene discussed is CTLA4; the disease is metastatic prostate carcinoma.