PDCD1 and neoplasm: Here, we rigorously validate tumor cell-intrinsic PD-1 functional expression by live B16-F10 melanoma cells and six distinct YUMM lines35,36 using multiple anti-PD-1 ab clones, recombinant (r)PD-L1, numerous negative and positive control cells of defined PD-1 expression level, and several independent assay systems.