CD274 and melanoma: Similar to PD-L1, the expression of PD-1 on T cells and its role to inhibit the immune system is well characterized, but recent studies have found intrinsic expression of PD-1 in tumor cells including melanoma (199), hepatic carcinoma cells (200), ovarian (201), bladder (201), lung (15, 202), and colorectal (15) cancer cells.