PDCD1 and neoplasm: Using this PD-1 ab clone and FVD employed in the aforementioned report to distinguish live versus dead cells, we confirmed surface PD-1 protein expression on both live (FVD−) and dead (FVD+) WT B16-F10 tumor cells (3.2 ± 10.8% vs. 6.6 ± 1.0%, mean ± s.e.m., n = 10, respectively, Fig. 2a).